Anyone who used Trillian before version 3 came out should give it a try now, its got a lot more features and less bugs than version 2.x had. I used to use AIM and MSN and hated both of them, especially AIM. I switched to Trillian and haven't switched back since, the only drawback to Trillian is that very rarely it has trouble direct connecting with people who are using older AIM/MSN clients, mainly AIM, so if the person you are talking to isn't using the most up to date AIM client, there may be slight problems with direct connecting/sending files, but again it is very rare when that happens. There is also a ton of skins available for it via the official site and Deviant Art if you are picky about that kind of thing.
Skype is a pretty neat program for chatting with one person via text or voice, but I have seen it have problems with voice conferences, that might have just been user error on the people I was talking to though. I used to use it for chatting while playing Star Wars Galaxies but switched to Teamspeak since it seems more suited for gaming chat.