I see I'm coming into this thread late, but I see no reason why we can't continue the discussion of campaign settings.
Here are my tho cents worth on the ones I'm familiar with that haven't been mentioned much, or not much:
Al-Qadim - can't believe no one mentioned this one. One of my favorites for many years. AQ is arabian adventures, and is set in the FR world of Toril.
PROS: AQ is rich in flavor. If you like the feel of arabian adventures, this one is for you. The classes are all unique to the setting, and there are no strange racial twists or rules oddities. There are numerous supplements, many of which are quite good, but the best part is that you only really need the original book, as it covers all the basics, and don't obfuscate the matter. Has a small, but loyal following on the net, and attempts have been made to translate it to 3rd edition.
CONS: 2nd edition, and is obviously out of print. It may be available .pdf from rpgnow, I don't know. If you stick to the one book, your better off, because a lot of the additional supplements got carried away, and complicated the core setting quite a bit, plus most are just unnecessary.
[bBluffside[/b] - already talked about a bit, I was one of the main authors of this city. Bluffside is a detailed city that is generic and can be placed in almost any setting.
PROS: Bluffside has a huge amount of detail, and the list of fleshed out npcs in the back is probably worth the price alone. Aside some minor nuances, it can be placed in almost any setting with ease - you just need a cliff overlooking an ocean. There are over a hundred richly detailed locales defined within the city, each with plot hooks, so the DM will never run out of ideas. There are also underlying metaplots that can spark the DM's creativity. I don't believe Bluffside is out of print. You can still order it from mysticeyegames.com.
CONS: I can safely say these since I am at least partially responsible.

Bluffside was literally written by a half a dozen people all contributing scads of locations, and crunchy bits. The gods were intentionally made generic to increase portability. But, because there was little synergy between the disparate writers, we ended up with some things that should have been completely left out, IMHO - Sixam and Nevae were added, not because they were crucial to the setting, but because of kitchen-sink syndrome. There are a few internal inconsistences in the book due to a lack of communication between authors, but most people will never notice them. If I were to redo Bluffside, I would firstly just replace gnomes with steam gnomes, and remove Nevae, Dragori, and Sixam. It's much more consistent with core that way, and less complicate.
The Deep/Dragon Sands - if you're interested in expanding the scope of Bluffside, you should consider checking out these products. Both are richly detailed expansions on deserts/sub-aquatic themes. Additionally both books contain non-setting rules that are applicable to any CS.