ichat seems to take almost no resources, which is super nice. it's also nice not to have to have the program open, and to have rendevous doing its thing (particularly nice on my school's campus where all the dorms are on the same part of the network).
AIM does kick ichat to the curb in terms of features, though. if ichat could work something out with its away messages, that might be enough to get me to use it again (though it has other issues, as well).
basically - ichat is really restricted in what you can do, but if you don't need much, it's great. if you want more features, you're going to have to go with AIM or the like.