Pathfinder is essentially the spiritual successor to 3.5 D&D for those folk protesting against the (real or imagined) atrocities done to the game by 4.0e.
Those who like it (myself included) describe the game as being 'essentially 3.5e but with a lot of things fixed' or made easier or more streamlined in one or another ways. According to what I've heard, Paizo actually makes the effort to listen and respond to feedback from their fanbase.
Those who dislike it (I haven't met one yet, but teh Interwebz are a bountiful source of information) complain that it is like they tried to fix some things wrong with 3.5e, but didn't make enough changes.
Things I like about it:
-First of all, I like that I can bring it to anybody who has played 3.5 and they can adapt to the rules without blinking an eye.
-Many things *have* been made streamlined--e.g. Instead of 4 skill points + 1/level you simply have 1 point per level and the difference is made up with a class-skill bonus that you automatically get for having ranks in the class skill, allowing for much easier cross-class dabbling; Instead of a hundred different rules for special attacks (Sunder, Disarm, etc.) you have a stat closely resembling the old Grapple Mod called 'Combat Manoeuvre Bonus' that covers all; Racial Favoured Classes done away with; Several skills (i.e. Stealth -> Move Si/Hide, Perception -> Spot/Listen/Search)
-Some awesome flavour things--the first one I ran into that began my steadily growing appreciation for the game was the Sorcerer Bloodlines: powers gained based on where your magic comes from, making them more than just wizards with fewer tricks more times a day (oh, and automatic Eschew Materials, because really, what Sorcerer wants to be inhibited by components from casting the magic that flows through his veins?).
-Some mechanics changed for easier access--Concentration is not longer a skill that your caster would be suicidal to ignore but no other class really bothers with but a stat automatically built into every caster; Spells no longer drain XP and permanently chucking you a level behind for the rest of the game but rather cost mucho coin.
In the end I could go on about the things that were changed and made fancier for a dreadfully long post, but the things that I pick out that appeal to me (like new class abilities adding variety, or cap-features [a la the Monk's 20th level ability] as incentive for characters to single-class it to the top for a change), but these same things won't necessarily be what appeals to everyone else, so instead I can leave a link to the Pathfinder System Reference Document and let those curious browse a bit for themselves.
d20pfsrd.com I recommend checking out classes first.
With that out of the way, I can point out that I do in fact enjoy other systems and Pathfinder is not my one and only true love. I also enjoy CJ Carella's WitchCraft, 3.5 (though I encourage a hybrid of 3.5 supplemental dealies with PF rules), d20 Modern (my first system =P), and I find that 4.0e has a lot of fun battle options despite its lack of roleplaying focus. Also, I haven't played (yet) but I have heard good things about and intend to one day partake in Shadowrun, Exalted, and 2e (the latter of which I almost played but for a premature game collapse).
EDIT: My elegant and finely crafted link was not so finely crafted--it should take you where you want to go now.