TL;DR - Ultimate Spheres of Power is a great system with many strengths. However, this compilation, and the handbook supplements it is derived from, suffer from lower editorial quality and attention to detail than the original Spheres of Power book. People who are familiar with the original should be aware that they may have a different experience with this product; I am not comfortable letting my players have free reign with options from this book like I was the original.
The original Spheres of Power book is my favorite Pathfinder product, not just 3rd party supplement. I like the Spheres magic system better than the Vancian magic system in Pathfinder, and think using Spheres improves both the mechanical and thematic enjoyment of magic. The system simultaneously delivers greatly increased flavor and freedom and generally more powerful magical effects while also removing many of the classic sore points with traditional D&D spells and actually improving game balance and ease of play. I would not GM a Pathfinder campaign without it, and I have players who wouldn't play without it.
All that said, the editorial quality of Spheres content decreased significantly after the core Spheres of Power book, and this compilation includes the potentially problematic content from the later expansions. Some were fixed during the playtest for this book, but after following the playtest and revisions closely, it's my opinion that the majority of the problems remain. Below are some of the recurring kinds of issues I encountered. These may not be a problem for every group, but buyers should be aware of these potential issues.
- Overly efficient action economy - There are many, many new talents in USOP that allows casting effects as a move, swift, free, or immediate action, including an entire sphere of free-action buffs. This is not relegated to support and defensive magic; there are many offensive effects that work the same way. There are also several spheres that allow applying two different effects, with separate saving throws, to the same area in a single cast. Very many of these are also cheap, often costing 1 extra spell point, rather than the 4 spell points Quicken Spell usually costs in Spheres.
- Overly large numerical bonuses and misuse of bonus types - There are multiple examples of this, but I'll just explain one particular example. The Devil, a motif talent from the Fate sphere, gives a +2 insight bonus to attack rolls and AC, that increases by 1 for every four caster levels, against enemies that have been previously studied as a free action. At low caster levels, only one target can be studied per round, but for every 5 caster levels, another enemy can be studied in a given round. This studying effect lasts for hours per caster level, so studying can occur before combat begins if possible, and there is no limit to how many enemies can be studied. (This duration also means that this buff can actually be cast long before combat and activated as a free action followed by an immediate action, making the motif talents one of the previously mentioned action economy issues.) From 5th level on, if the caster waits only a single round before activating the buff, four creatures can be studied, making this buff usually effective against most enemies in most combats. In Spheres of Power, caster level can grow to 25 easily (including a +5 bonus from a magic item that is the equivalent of a +5 weapon for a caster), meaning this talent provides up to a +8 insight bonus to both attack rolls and AC against most enemies you'll encounter. That is already a larger buff numerically than exists previously in Spheres or base Pathfinder. It is also an insight bonus, which are doled out very carefully in official Pathfinder material. There are no large situation-independent insight buffs given by any of the thousands of Vancian spells, which is not an accident. Major spell bonuses are given in the form of enhancement, size, and morale bonuses to limit how much stacking can occur. The Spheres system already has buffs of these kinds, and then added effects like this that stacks with it. This can extremely imbalance the system's mathematical expections and result in a poor play experience, and I highly recommend you do not use talents that fit this category at high levels.
- Potentially inappropriate narrative effects - Spheres had the very useful innovation of Advanced Talents, talents that fundamentally changed how the game was played and might not be appropriate for some campaigns or groups, such as long-distance teleportation, resurrection magic, scrying and other very powerful divinations, etc. They are explicitly opt-in and not assumed to available to players unless the GM makes clear otherwise. This book has a few effects that I strongly feel belong in the category, but aren't put there and therefore may take groups by surprise in unfortunate ways during play. A good example is The Devil motif, whose studying effect reveals a fairly accurate range for a creature's CR, which may be a really interesting effect in some games, but a complete plot-ruiner in others. Another reveals a person's caster level and remaining spell points. A clever GM or one willing to bend the rules can of course work around such an issue, but such effects really should be called out ahead of time, and the core book was far better about doing so than the expansion material.
[3 of 5 Stars!]