I'm starting to get into Soundcloud a bit more and it's been pretty positive so far.
Everything streams well - I'm on a stone-age 'broadband' connection and everything plays without stuttering
The free account allows a user to have 2 hours of music and from what I can make out, that could be in any number of formats - uncompressed wav or whatever rate mp3s. You're not limited by file size, but by duration.
The timeline comments idea is quite unique (and useful).
Pity its player can't be embedded in this forum - well I can't figure it out, that's not to say it can't be done !
Mixcloud is more of a cloudcast/radio thing - different but still interesting, though.