I've yet to run into any major problems with iChat, however I found one semi-annoying one. If you set your status to the default "Away," no Windows AIM users (maybe other clients as well, only tried with Windows AIM) can read your away message. Instead, only this message pops up:
The text has characters that cannot be
displayed with this system's fonts, and instead
are displayed with substitute characters.
I still don't know what my standard away message says...