Carnipelago is very impressive, and one of the tightest, best-coded and described MU's I've ever seen. It's based on a totally original theme that's part David Lynch, part Fellini, part Asimov: a world where generations ago a spaceship of carnival performers crash-landed and started a whole civilization. Now people from off-world have become part of the mix, and are trying to figure out how to fit into this strange world. Imagine Six Flags mixed with a weird New-Orleans-style carnival, and an island atmosphere all rolled into one. The place is deliciously user-friendly, but challenging and quite large already even though it's relatively new. And the variety of objects, gadgets, panoramas, stories, etc is exquisite. If you prefer huge throngs of users, don't go. But if you want to get in on the ground level of a very new, mature, subtley formed MU concept, this is a great place to do it.