I've been using trillian for now, as most of the others I've used don't use the push messaging, and we all know that's a horrible battery drain without that activated.
The thing I don't like though is that if I lose data for a second, and reconnect, it triggers all messages that I've read already as new IMs and sets the notification, which is quite annoying.
I'll look into Beejive. Trillian is the nicest app I've used so far, but it's really picky with data connections. There are times where it just doesn't want to let me log in at all, even though I can get a working data connection just fine.