If D&D 3e/3.5 was a game you enjoyed and thought was the bees knees, then Pathfinder will likely be a game you love. It did clean up some of the messier bits of 3.5 (namely unarmed combat, grapples, some skill consolidation, etc), and made some classes more interesting (the sorcerer especially benefits). The cleric domain powers and abilities are also much better than those found in 3.5. The XP system for awarding experience is better too- the CR/EL stuff is largely gone now. Any of your old 3.5 stuff should be easily convertable to Pathfinder as well!
However, Pathfinder also has retained all of the 3.5 design flaws. Magic item Xmas tree (reliance on magic items) is still a big problem, as is the complete dominance of caster classes. Skill points are still few and far between for most classes, and massive bonuses to d20 rolls that largely make the d20 roll irrelevant are still there. Armor class doesn't scale with level, and there are numerous small changes to feats and spells that mean you really have to carefully reread everything to understand how the system works now. Some folks love the feat trees and prereqs for Prestige Classes, and enjoy planning out a character from level 1 to 20; but for my group, it was a turn-off to have to pre-plan everything to get a desired feat or PrC instead of letting the character grow over time in an organic way. Finally, DM prep time hasn't been improved upon at all- the guy who ran it was constantly frustrated by his prep time and having to double-check every spell, feat, and magic item he thought he knew from 3.5. For my group, things were a bit too regimented and rules-opaque under Pathfinder for our enjoyment (a problem we also had with 3.5).
My group tried Pathfinder for about six months after the beta came out, and tried a short campaign with our updated beta characters when the main rulebook was released last summer. I played a human sorcerer up to level 9, and while the character was more flavorful, in the end our group had the same issues with Pathfinder as we did with 3.5, and we dropped it. We've been playing 4e the entire time we were playing Pathfinder, and the difference isn't even close- for my group, 4e is a far superior game. Each group is different though, and Pathfinder may fit your gaming style better.
Finally (and this is the area Paizo really kicks ass), you're going to have a LOT of great adventures to work with. I'm not quite as big of a fan of the Adventure Paths as some folks (they feel a little too railroady to me), but I do LOVE their 32 page modules, and have used several (or parts of them) in my 4e campaigns. Good luck, and enjoy your gaming!
