Disclaimer: I backed the Kickstarter for this project and followed it since the beginning and participated in the playtesting of this material.
One thing that I would like to say upfront is that if you are ONLY planning to purchase this product in hopes that it will make martials on-par with Tier 1 classes (such as the Wizard, Cleric, and Druid), DON'T. Even if your game has replaced core vancian spellcasting with spherecasting, Spheres of Power is still without a doubt superior to martials using Spheres of Might. It has been discussed at length that it wasn't the mission of Spheres of Might to fix martials in that regard.
What I will say this product does do, is allow you to build martials who are defined not so much by their class, but how you build them, and it all starts with Martial Traditions.
In Core pathfinder, all too often you will find GM's and Players who are under the false impression that in-order to play a specific character concept, you must have levels in a base class or prestige class which matches the name. For example, if you want to play a ninja, you must have levels in the ninja class; if you want to play a samurai, you must have levels in the samurai class; if you want to play a druid, you must have levels in the druid class, etc.
Spheres of Power (the older companion product), throws this notion out the window with the use of Casting Traditions. With casting traditions you can play any spherecasting class and just choose the relevant casting tradition. For example, you could be an Armorist with the druidic casting tradition, a Hedgewitch with the druidic casting tradition, or an Incanter with the druidic casting tradition; it makes no difference.
Spheres of Might, does the same thing for martial characters with the use of Martial Traditions. Which allows you to define your character even further by defining just how your character was trained. Where you a knight? A thief? A gladiator? There are martial traditions for these and 30+ more, while also including guidelines to creating your own. And that is just the beginning.
After picking your martial tradition (which determines bonus starting proficiencies and starting combat spheres), you can further build, define, and expand your character even further by picking up spheres and talents from a list of 20+ combat spheres which cover aspects such as Alchemy, Beastmastery, Dual Wielding, Sniping, and Scouting (just to name a few).
Spheres of Might also includes Legendary Talents (which like Advanced Talents from Spheres of Power) must be approved individually by a GM. Personally, for a number of legendary talents, I feel they were locked behind a specific level unnecessarily. Most notably legendary talents such as Sever, which allows for the amputation of limbs (but is locked behind a BAB prerequisite of +11). The problem I see with this is that it infers that soldiers in war do not experience limb loss unless fighting something with 11 or more HD. It also infers that a medieval surgeons cannot amputate limbs before 11th level. Ofcourse the authors have repeatively given their explanation for such saying that it is because they don't want players to lose limbs before magic is available which can restore the condition (which I feel is a weak argument, seeing that death is a condition that players face at 1st level without affordable means or restoring that condition). However, these small gripes are not ones that I consider strong enough to reduce my rating of this product significantly.
Spheres of Might also offers a wide range of new base classes (and archetypes) which utilize Spheres of Might to its fullest potential, all of which I feel are fun alternatives to a number of Paizo Classes. For example, the Scholar class could easily fill the role of a number of classes (alchemist, bard, cleric, or wizard); whereas the rogue class could easily be replaced by the new Conscript, Striker, or Technician class (depending upon the type of rogue built).
For GM's Spheres of Might includes an array of pre-statted monsters ranging from CR 1-20, aswell as fast and easy guidelines for giving Martial Traditions to monsters.
Personally, I feel that Spheres of Might shines the most when combined with Spheres of Power, as they compliment each other nicely by lowering the power of casters, while raising the utility of martials; and while Spherecasters are without a doubt still superior to Spheremartials, this product does allow a martial to more fully enjoy his contribution to the game table.
[5 of 5 Stars!]