The funny thing about the power attack in Pathfinder is that at least for my characters it is more useful. I don't often take two handed sword swingers so the -1 to attack and the +2 to damage works out better for me. In 3.5 you only get that with a 2-hander. Pathfinder you get -1 to attack and +3 to damage with a 2-hander so that's not too shabby.
Making it a flat penalty and bonus with every +4 to your base attack seems unnecessary, thought I do like it capped(in my games I have a harder time with the fighters running amock, not the casters). I think we have house ruled that to allow you to pick your penalty and add that amount back to your damage bonus. We still are using the cap (The -1 for every +4 BAB)....For now anyway, I've heard some squawking on that from some players. Worked out well for me the other night. I tore up a fighter PC with a NPC using the new power attack.
Really the complaint is from Combat Expert. It only adds a +1 dodge bonus for every +4 BAB. Seems like a spell caster nerf to me. In 3.5 with combat expert you could shove your entire BAB into AC, and why not? Your casting a spell anyway, so you don't need it for attacking (ya unless your doing a touch spell I know). This will slow down that abuse.
Cleave is better. Instead of getting another attack when you kill a foe. You get another attack at your full BAB if you hit your first target, then you can target someone adjacent and attack. So at low level it's a nice two-fer if your fighting multiple baddies.
And I really like the Combat Maneuvers even though they are generally harder to get off in combat. One mechanic, one roll. It's way faster. And the improved feats now give a bonus to attacking and defending against the maneuver. Epically failing a combat maneuver can be a bad thing. They include grappling in with the Combat Maneuvers as well so that rocks.
So far everything, with some tweaking, has been compatible with my 3.5 stuff. Some things are not needed anymore, and you will have to make some on they fly decisions about what skills are class skills for the old 3.5 prestige classes and stuff like that. When new books from the Pathfinder people come out I bet they address some of the more popular 3.5 prestige classes and add them into their game. I'm running a 3.5 adventure, but useing the pathfinder rules, and so far no hitches. I converted some of the major NPC's in the adventure, and they turned out a little bit better but nothing major. I'll be doing some higher level ones in the future.