These are just a few notes explaining the tier rankings and other considerations for sphere casters and practitioners.
Spheres of Power and Spheres of Might are both popular third-party supplements for Pathfinder from Drop Dead Studios. They provide an alternative to vancian spellcasting and traditional martial classes through a talent system akin to spells and feats. The primary benefit of this system is that it allows for more build diversity and flexibility, often giving players more options for how to build their characters. This doesn't necessarily increase the power or versatility of spheres character compared to normal characters, but it does give players more control (especially in the narrative sense).
Generally, sphere casters are tier 3, with low casters coming in at tier 3.5. In other words, they're noticeably weaker and less versatile than vancian casters, but they can still specialize around a theme. Many advanced talents are powerful enough to warrant a tier increase, with the ceiling for sphere casters probably being tier 2. Make sure to run any advanced talents you plan to take by the GM.
Spheres of Might practitioners are almost all tier 3.5, since talents essentially function as bonus feats (and they get a lot of them). This makes them more versatile than most fighters, rogues, or traditional martial characters, but not as strong as martial initiators. Some legendary talents warrant a half tier increase, but the ceiling for practitioners is probably tier 3.
Spheres of Power claims that sphere abilities are not spells, and shouldn't interact with existing rules as though they were spells. Yet they are spells in all but name. For Ledge Campaigns, you can go ahead and treat them like spells when doing so would make sense. You should still use the existing rules for sphere / vancian interactions, but certain options (such as metamagic feats) can be applied to spherecasting as appropriate. Sometimes this doesn't make sense, for example the Heighten metamagic feat does nothing for sphere spells without a way to break limits.
The spheres wiki has several recommendations for gestalting spheres characters. Follow those rules where they're compatible with our rules. Of note, multiclass and gestalted practitioners receive bonus discipline talents for every martial class after the first.
When gestalting two spherecasters as part of the same tradition, only add your spellcasting ability modifier to your spell points once. So, ignoring any traditions or sources of bonus spell points, a 4th level gestalt full sphere caster would have 4 + 4 + mod spell points.
I'm open to players who want to make their own spellcasting or martial traditions.
If you would ever gain a bonus sphere talent for a sphere that you already have, then you may gain a bonus sphere talent in that sphere instead. This seems to be the intent, but it's not always explicitly stated.
The aristeia system from the Study and Practice supplement will eventually be incorporated into the fate point system. Oaths and boons are allowable when they make sense (don't push these too hard).
Researching new spells or using techniques for spherecasters should probably increase their tier by 1.
Note that the Ultimate Spheres of Power book has been released. We'll continue to use the most up-to-date rules for the system.