I guess I'll put my two cents on the table too.
Forgotten Realms: The production values of the 3e FR stuff are the highest I've seen anywhere. The campaign setting book is truly a thing of beauty, from cover to cover. And that map! However, the setting is so high magic and so detailed that it feels to me like there isn't anything left to do in that world. And then there is the problem of finding players who won't think they know exactly how everything is supposed to be. I own most of the FR 3e stuff, and it's fun to mine for ideas, but I doubt I'll ever run a game in it.
Kingdoms of Kalamar: This is the second most beautiful collection of books. The atlas is one of the coolest things I've ever seen. I love the Orc book and the Hobgoblin book too. I think I have everything published for 3e Kalamar except for the recent book about pirates. The magic level is lower than FR, and in fact, you could run it with nothing but clerical magic if you wanted to. I love this setting, but I have yet to run a game in it, and until just the other day, I hadn't figured out what was holding me back. Like the realms, there is nothing left to explore. All the discoveries have been made. Now, there are a number of modules available for it, so you can just tie all of them together for a story, but if your players want to wander off, you might have a bit of a problem thinking of cool things for them to run across. At least I know I would. But if you're interested in Kalamar, talk to Wicht.
Freeport: Pirates and lizardmen and corruption, oh my! If you're into the pirate genre, I don't think you can do better than Freeport. Plus the setting is small enough that you can tack it onto just about any other world, if you decide you've had enough salty dogs in your game. I used a slightly altered Freeport in my previous campaign, and it went pretty well. One of these days, I will set an entire campaign in Freeport. It's good fun, matey!
Oriental Adventures/Rokugan: Nicely done, but doesn't interest me much. I'm not sure why I bought all the books. I want to like it, but I just don't, for some reason.
Scarred Lands: I was really excited about this setting when I read Relics & Rituals. That's an amazingly evocative book. I like some of the monsters from CC1 and CC2. But the campaign setting just killed it for me. As I read it, I kept thinking 'yeah, right' and 'oh, puleeze'. I just couldn't take it seriously or suspend my disbelief. I know many people like it, but I just don't.
Greyhawk: There's no there there. And the names are goofy. Furyondy? Verbobonc? How am I supposed to say Verbobonc without giggling? But even if the names didn't make me roll my eyes, Greyhawk is the campaign setting version of abandonware. So I couldn't recommend it.
Midnight: I want to like this one. I think it has some really cool ideas, especially the races. But I can't DM a game where there is no hope, and more than one of my players has said they will not play in it, because it's too depressing. We game for fun, not to work ourselves into a funk. I know other people think it's really cool, so you should probably listen to them instead of me.
Wilderlands: Just got this, so I can't say much. Except that I'm in love. I knew by page 15 that it was the campaign setting I've been waiting for. I love that there is plenty left to explore. I like the crunchy bits. I like that there is a Toad God, and a Goddess of Deepwater Fishes, and a Demon God of Disease. Ignorant superstitous people everywhere, amid the ruins of an ancient empire and marauding monsters. Evil rulers. Crime and disease ridden cities. What's not to love? I think I'm going to shove Freeport in there somewhere, and then my life will be complete.