Core Pathfinder is an improvement over 3.5. That's a fairly simple equation. Not only are the classes a bit more interesting, but the consolidation of mechanics into Combat Maneuvers is great, being able to build encounters based on XP budgets is really handy, and some of the simple templates, like Advanced and Giant are wonderful for boosting something that you want into an appropriate challenge without much hassle.
I can't put my finger on it perfectly, but the way the Bestiary monsters are laid out makes it very easy for me to use them out of the book as is, in a way I could never get used to doing with 3.5 books. I'm sure part of it is the fact that there are rarely multiple monsters on the same page, but something is visually pleasing and easy to follow about the layout of the monsters.
When it comes to rules clarifications . . . the Paizo staff are on the forums often and communicate with the fanbase . . . but they seem to either give contradictory information or give really hesitant answers that don't fully fix problems. In other words, unless you are going to get into organized play, figure out how you want to resolve something, tell your players, and don't wait for the official ruling, because you might get none or multiple, contradictory rulings.
Now, depending on what bugs you about 3.5, here is where some of my 3.5 issues began to rear their heads again.
The Advanced Player's Guide is not, overall, a bad book, but there are some landmines in it. The Summoner may not be as bad as it seems, but its such a complex class that its very easy for someone to screw up, not realize it, and create some really wonky eidolon combinations. The point based creation method isn't the friendliest way to go compared to, say, animal companions.
New spells in the APG also open up some ground that hasn't been trod in the core rulebooks, and does some things that 3.5 did in introducing "iconic" abilities early with lower level spells. For example, Vanish allows invisibility from day one in the campaign for 1 round/level. Also, some spells don't seem nearly as problematic until they get used a lot. The pit spells have become my least favorite chain of spells ("well, it was going to be hard for the whole party to function in this encounter given the size of the room and the number of enemies, but now that there is a 10 x 10 pit in the room, half the party should go out and have pizza until the pit spell expires").
A friend of mine picked up Ultimate Magic to see if I would allow it into the campaign, and I'm seeing a lot of headaches in that book. When Paizo is on, they are really, really good at d20. When they are bad they are just as bad as WOTC in its worst 3.5 books.
I'd tell you to look on the PRD, but thus far, they haven't gotten the Bestiary 2 or Ultimate Magic in there yet, even though they have said that its a priority, just not as much of a priority as getting more books ready for Gen Con.