microsoft doesn't like to share it's protocols/servers with third parties (hence xbox live)
so when you'll use adium, you'll lag ... and when you send files, it'll be 100 times slower. sometimes you'll chat and it just won't send.
in terms of IM (instant messege) the best choices for mac/windows in 2012 are skype and facebook
in canada where MSN was #1 IM, no one touches it anymore... everyone moved to skype/facebook
ichat is useless, it only works with yahoo/google/macs which no one cares about
skype or facebook.