I think the best thing about Pathfinder is it's an ACTIVE rules line, whereas 3.5 is now no longer supported.
I tried Pathfinder, and I liked some of the changes they made, but it failed to address some of the problem areas.
For example, they changed how Grapple works, but it's still a combat maneuvre I avoid purely for the rules hassle.
In the end, I decided to stay with 3.5 mostly because the Pathfinder rules are too close to the 3.5 rules, resulting in me second-guessing myself on every rule (am I remembering the 3.5 rule or the Pathfinder one, and/or are they really different?) and causing more rule-lookups then when I actually started playing 3.5.
My conclusion:
I think Pathfinder is better than 3.5, although I might not have played it enough to make a real comparison.
I also think using rules that may still get errata and FAQ entries, or at the very least Customer Support is preferrable to a 'dead' (as in, no longer supported) ruleset.
Because Pathfinder is so close to 3.5, you can probably use some of the splatbooks from 3.5 without too much problems, although you have to take the changed core rules into account.
However, if you are already comfortable with the 3.5 rules, I would suggest staying away from Pathfinder, because of the rules headache mentioned above.