I don't get the popularity of Skype. The voice quality is horrible, even with just two people
It's fairly dependend on your internet connection. Personally I've never had any problems with it.
(I use Skype as my main IM program, both for webcam/voice calls, but also just for plain vanilla text chats.)
I'm sure that AIM and MSN are 'capable' too, but MS's habit of integrating their junk too deep with the OS and AOL's history of screwing with OS settings they shouldn't even be close to means I don't even consider either one unless there are no viable alternatives available at all.