As I have said elsewhere I have tried every other IM app for the phone as they came out. I was psyched to use AIM when it came out, but after two days I found it simply not capable of being a replacement for my desktop IM'ing, and if you don't know, trying to im from multiple locations is fraught with all kinds of issues.
Not until Beejive came along did I find a real solution that allows my iphone to be my main IM device. I don't run IM on anything else now, and I use it with icq, mobileme, googletalk, msn at the same time.
It works, and it works well. If you actually want to be able to use IM on your iphone as IM is intended to be used, then Beejive is the only current choice. Otherwise the other apps are novelties that are fun to play with but not actually useful for fully functioning instant messaging.