Merrick, I played 3.5 up to level 40, and I'm currently running a custom (no Adventure Path) campaign with an high number of players (up to 12) Pathfinder only. Level 12-15.
Overall, the game is an huge improvement of 3.5. classes are more interesting and versatile. the quality of the books (I own core, bestiary 1 and 2, APG and GMG, and I play with those only) is high too, with few exceptions. It remains a 3.5 game - so if you really hate the 3.5 style, avoid it.
Disparity in power between classes has been lessened because
1) a lot of SoD spells have been changed. This has been somehow reversed by a feat tree in APG, but works for a spell only.
2) high level meleers and ranged attackers are SCARY damage wise AND are in general capable of more than few trick based on control (fear, maneuvers). if you want a spell-resistant meleer, I humbly suggest a barbarian, not a fighter.
Arcane Casters are still very strong, but melee, generally speaking, ends the combat - arcane casters change the flow (generally speaking - my players have a blaster sorcerer and a control focused fighter, and they rock too!).
My combats are long, but consider the number of players a standard party should do fine (ask other people). I generally make my own monsters only if humanoids (say, Goblin Witches, Hobgoblin Two-handed fighters) but you have a good array of opponents by bestiary only. I want to point out that the quality of bestiary 2 is equal, if not superior to the 1.
Bestiary has a page with guidelines for monster creation (damage, HP, AC, Saves) I use it to create on-the fly challenges, refluffing (someone could say "4th edition style").
Overall, swingy elements remain - people could consider it a bug, but for me it's a feature. Moreover, a good x3 critical could mean end of combat, hence a shorter combat.
In high level play, elements of "spell used to counter a spell to counter a spell to counter a spell" remain. As an example, in the last adventure the players discovered an attempt of scrying from the BBEG thanks to the wizard casting detect scrying. Teleport and similar stuff remains. My group love this, because it's a sign of the power level change. i want to avoid telport in a place? team Evil cleric casts Forbiddance. It depends from your tastes...
At the end, like 3.5, I just put up thing in a way that people don't ruin my plot using a powerful spell or using a powerful feat/skill combo, but are SUPPOSED to do this, otherwise they are defeated.
I hope it can be useful, feel free of ask anything, specific (of the campaing, or of a class).
