Ultimate Spheres of Power is a massive book with more than 600 pages that updates to the alternate magic system presented in Spheres of Power as well as the handbook project that expanded all 20 spheres. The book also includes materials from Spheres Apocrypha and other Spheres supplements that were published before its kickstarter. I have backed the handbook project for spheres on patreon as well as this book on kickstarter. I also participated in the feedback and discussions during the playtest of Ultimate Spheres of Power and its updated rules.
First off, when I picked up the original Spheres of Power back in 2015 I instantly fell in love with it and I now find little interest in playing pathfinder without it. The system could be used both together with or instead of normal vancian casting, and excelled at producing spellcasters based around a theme. With Ultimate Spheres of Power, this still holds true.
The idea is that you choose a couple of spheres which grant at will abilities you can augment by spending spell points. And then learn talents that expand those spheres, either by granting new abilities or granting effects that modify existing abilities like increased range or the ability to target an area. Abilities that significantly power up an ability such as [mass] talents that change a power from single target to multitarget or improve action economy with a single ability typically come with a spell point cost in addition to the cost of spending one of your limited talent choices.
The book is so massive that I won't be able to go in depth on all the options, but I will try to give some highlights of the options presented.
Casting Traditions: Instead of the arcane/divine/psychic divide of the base pathfinder, each spellcaster requires a casting tradition which specifies details about how their magic works. This includes choosing a casting ability modifier (usually Int, Wis, or Cha), general drawbacks (does your magic require Somatic and Verbal components? Does it require a focus like a holy symbol, ring, wand, or metal weapon? Does your magic cause a horrible addiction, drain your life energy, cause pain, require you to play music, or drain energy from the land around you? And many more options), and boons (such as having an easier time maintaining spells with concentration, ability to overchannel to temporarily increase your power, or getting more spell points to fuel your magic). Casting Traditions should ideally be determined in a cooperation between the GM and the Player, but also serves as a tool for the GM to customize his world. Casting Traditions can also be assigned as arcane/divine/psychic or something else entirely if desired to increase the transparency with vancian casting.
The Spheres: The 22 spheres Alteration, Blood, Conjuration, Creation, Dark, Death, Destruction, Divination, Enhancement, Fallen Fey, Fate, Illusion, Life, Light, Mind, Nature, Protection, Telekinesis, Time, War, Warp, Weather, are all distinct and flavourful with mechanics that support the theme and allow for both specialization and diversity. The chapter with all the spheres and their basic talents is around 160 pages giving lots of options.
Advanced Talents: Magic abilities with significant impact on the setting are presented in a separate list of advanced talents. This is where you can find long range teleportation, scrying, creating 2 mile radius darkness, creating instantaneous and permanent effects, and so on. Depending on the style of game, a GM can choose to allow or ban advanced talents. Even if they are allowed, a player should still inform the GM of any advanced talent they take as even in high magic settings where most options would be fine, some can still have major unwanted impact and with well over 30 pages of advanced talents they might not be familiar with all of them.
The Somewhat More Neutral
Sphere-Specific Drawbacks: Whenever a character gains a new sphere, they may choose one or more sphere-specific drawbacks for that sphere. In Ultimate Spheres of Power spheres gained through temporary talents cannot benefit from sphere-specific drawbacks and temporary talents cannot be used to buy off a sphere-specific drawback. The way these drawbacks work you lose access to an ability in the base sphere or limit that ability in some way in order to get a bonus talent in that sphere. This is a central and important mechanic when it comes to getting your concept to line up with your mechanics, but also a somewhat fiddly mechanic that can yield a lot of extra versatility if optimized.
Classes: While the classes are generally good, I find that most are either a bit bland or a build your own class type of class. The Incanter is perhaps my favorite class, but it is essentially a generic sphere caster who is specialized in having many magic talents. They did mostly fix the issue of dipping into the Incanter as they can no longer gain all their specialization options at 1st level, and the sphere specialization options now mirror the Conscript from Spheres of Might granting a special ability at 3rd level, 8th level, and 20th level making the option feel more unique compared to the original version. Similarly the Hedgewitch is a generic mid-caster and the Mageknight is a generic low-casting gish, but they are well made and interesting. The Elementalist stands out as everything Paizo's Kineticist wishes it could be, namely an easy to use blaster with mechanics that are easy to grasp and ability to pick up some thematic utility options. Correct application of the Casting Tradition and use of the Nature Sphere and you will have a great elemental bender.
Archetypes: With about 120 pages of archetypes, there is a lot to choose from and the quality will vary. A few archetypes such as the dual-blooded sorcerer were removed in Ultimate Spheres of Power as they essentially traded out nothing for a lot of power, while many of the generic sphere-archetypes for pathfinder classes remain pretty bland. There are however also a lot of inspired archetypes that change up classes in fun and interesting ways.
Transparency: Ultimate Spheres uses something they call magic skill bonus and magic skill defense in place of caster level checks in base Pathfinder. While caster level determines the scaling of abilities and DCs of sphere abilities. This can get a little confusing for players who are used to Pathfinder and d&d. For the most part the systems interact with each other in the way you would expect, and basic rules are specified in chapter 1 while specific instances of how the sphere talents interact with Paizo spells are called out in the talents where it is relevant. It still happens that you run into unclear interactions.
While the original Spheres of Power was clearly a downgrade in power compared to vancian casting, especially at higher levels. This was mostly due to the loss in versatility. Even in the original, a sphere caster could pull off abilities that a vancian caster would need to wait until a higher level for (such as short range teleportation, or a mass charm monster) due to the nature of specializing in a single trick by investing all your talents into that trick. The possibility of creating this type of one-trick pony who has one trick that wins any encounter where it applies but is useless if it doesn't apply still exists in Ultimate Spheres of Power.
Ultimate Spheres of Power also comes with a massive increase in options, which has led to more powerful options. By dipping around, it is possible to gain a multitude of unusual bonus types that in some cases still scale faster than they should as well as unanticipated synergies. One example would be using the Community-Minded trait from Paizo with the War Sphere using the sphere-specific Battle Manipulation and Alternate Rally (for Empower). This allows the sphere caster for 1 talent and a trait at 1st level to as an immediate action spend a spell point to grant themself (as you are your own ally in pathfinder) or an ally within 30 feet of you a +4 morale bonus to either attack rolls and cmb, ac and cmd, saving throws, or concentration checks and checks against spell resistance for 3 rounds. While morale bonus is one of the common bonus types that matter, this together with other dedicated dips for either action efficient or long term buffs of different types can shake up math to the point where a buffed character makes the d20 rng irrelevant.
The increase in options has also led to an increase of save or lose abilities in Ultimate Spheres. While these have existed in base pathfinder and in the original Spheres of Power as well, they are much easier to get or stumble into in Ultimate Spheres. Due to how implements (magic items that increase your caster level and thus also save DC) work as well as how sphere casters can usually always operates at or near full power for as long as they have spell points, combined with the ease of making the save or lose ability multitarget, means spheres casters can shut down combat encounters just as hard as a vancian caster using their best spells.
In the end, my opinion is that while a caster using Ultimate Spheres is less versatile than a vancian caster, a vanilla paizo caster will usually find that they have less raw power compared to a sphere caster's best tricks. This becomes even more noticeable with unlimited access to advanced talents (which I would not recommend), as you have easy demon summoning in Conjuration that can match planar binding, the ability to retrain your whole character using Complete Reversion from the Time sphere, or creating a 2-mile radius area of death using Dark, Light, or Weather spheres. Access to advanced talents with GM approval is still needed if you want staple abilities like long range teleportation, or the ability to raise dead.
While the balance of Ultimate Spheres requires more GM oversight than the base Spheres of Power, I find that the rules updates and additional content is an improvement of an already amazing system. I personally will not GM Pathfinder without allowing spheres, though depending upon the style of game I might insist upon only using casting traditions with significant drawbacks and disallowing advanced talents. As a player, unless my concept is specifically based on a d&d or pathfinder style character I find it much easier to realize it using spheres and to adjust the character to be more in line with the rest of the party and setting.
As such I will give Ultimate Spheres five stars and a strong recommendation to anyone who plays pathfinder, and especially anyone who is tired of vancian casting and would like to try out some new options.