These are just a few notes explaining the tier rankings and other considerations for sphere casters, practitioners, and operatives.

Spheres of Power, Spheres of Might, and (now) Spheres of Guile are popular third-party supplements for Pathfinder from Drop Dead Studios.  They provide an alternative to vancian spellcasting, traditional martial classes, and intrigue-oriented characters 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 Guile operatives are almost all tier 3.5 as well, with some dropping to tier 4.5 or 4.  These are provisional ratings while we try out the system, but in general operatives seem to be on the weaker side of the three subsystems.  I noticed while evaluating the classes that they tend to get a large number of total talents (36 for the Envoy, for example), which weaker on average than Spheres of Might, gives them a fair number of options.  The plan talents also look potentially abusable, so I'll be keeping an eye on those.

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.  Operatives also gain additional trade talents when gestalting.  Since it's not on the wiki yet, here's the relevant text: "When using the Gestalt rules from the High Magic Handbook, a character selects a trade tradition for only one of their classes, but gains an additional (trade) talent. Any new classes the character takes in place of either of their original classes grant a (trade) talent as normal."

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, martial, or trade 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.

2023 Note: I've reviewed Spheres of Guile, Legends of the Spheres, and re-evaluated the other spheres classes and the Origins talents/spheres.  The tables should now be accurate.  We decided not to allow vancian/spheres transparency for now, but we may review this later.  I've also reviewed a number of various rule variants (aka optional rules):

• Associated Skills: Generally, I'd prefer not to allow skill ranks to exceed the normal maximum.  In cases where that's explicitly the primary function of an option, it's fine, but not as an incidental (and potentially abusable) side effect.  So we'll be using the Associated Skills variant to change ranks in excess of max to competence bonuses.
• Combat and Magic Talent Substitution: I'd rather not go with this considering this is already a gestalt game.
• Origin Talents and Skill Spheres: We'll be allowing Origins talents/spheres, so we'll be allowing this variant as well.
• Utility Talents in Non-Skill Spheres: I'm ok with this, gives some backwards compatibility.
• Unlocked Leverage with Combat Spheres: Let's go with the new Combatant's Tenacity talent option.
• Utilitarian Talent Progression: We can let this be an option available to players.
• Monsters, Skill Talents, and Trade Traditions: Sure. This is GM territory anyway.
• Gestalt and Trade Traditions: See above.  We're definitely using this.
• Leverage as a Daily Pool: I'd like to play with this before making a decision on changing leverage.
• Leverage for Improvisation: Same as above.
• Uncontrolled Emotions: I rarely call for this, but I have in the past so it makes sense to include.
• Skill Spheres and Techniques: Yes
• Degrees of Success: We've informally been doing this anyway.
• Skill Modifier Variants: Let's not cap things or limit bonus types but use the base stat ranges instead.  I'd be willing to consider temporary skill leverage when players would hit the cap of the base stat ranges.
• Unchained Skills: We're already doing background skills, but I think I'd be willing to follow their recommendations for giving free access to the 5 rank unchained skill unlocks (and then compensating the unchained rogue appropriately).
• Replacing Opposed Rolls: I'd prefer not to do this, but would consider it if someone wants it.
• Scaling Diplomacy DCs: We've informally been doing this anyway.
• Variant Disguise Modifiers: Worth consideration, I'll have to compare later.
• Scaling Lift Item: Sure, this looks decent enough.
• Dynamic Planning: More or less what we've been doing already.
• Light Body Technique: Not quite the campaign for this.  Maybe only on Eastern characters?
• Martial Contacts: This is more or less already a thing within the setting.
• Greater Compatibility Optional Rule: This is printed for Akashic Characters, but could work more generally.  I'll allow it.

See also:

http://spheresofpower.wikidot.com/archetype-rules
http://spheresofpower.wikidot.com/using-spheres-of-power#toc41