I find that with AIM and iChat, it's hit or miss.
My pc friend has a camera, as do I on my mac. I can infrequently have a videochat with her, it'doesn't work every time.
On Yahoo IM, the videochat works rather flawlessly. Unless the flaw is the video quality. it's pretty crappy.
In other scenarios (one-way video chats with a PC person, where my video is what's showing), iChat generally works well.