Do you have particular cities in mind? I don't think I could handle living in, say, NYC... but I do currently live in the downtown area of a considerably smaller (but still populated) city and it's pretty great. :-)
May I start with Barcelona? Or any Australian city. I suppose it depends how one defines city. I find the disconnection with land, with earthiness, far too oppressive. I met a very chunky American girl, lips the size of life-rafts, who flabbed at me, "Hey, ya know, my favourite tree is concrete." Good for her but it's not me.
I dislike most cities for anything except short visits, but there are some I never get tired of. London, for example. York. New Orleans--though I suspect it will never be the same after Katrina as it was before--was one of the most unique places I'd ever been. It doesn't even *feel* like an American city. Montreal. Adelaide.
Los Angeles is like another world. It's a fun place but I love coming back to reality when I leave it, too.
Again, not a big fan of cities, but every one has its own flavor, and some are pretty tasty. :)