[skorgu]

If you're on BIS it's just a very well implemented IM system. Logging in doesn't exist as a concept, messages are just sent like SMS. It provides sent, delivered and received notifications for each message and lets you do all the normal IM stuff like sending images, etc in relatively straightforward way. Honestly it's head and shoulders above AIM, gtalk, etc at least on my Tour. The add-a-contact with the barcode is nice too.

If you're on a BES I believe it ties in with Exchange's GAL so any contact the BES knows about is just available without you having to discover it.

The silly-named cnectd [1] is a cross-platform (iphone/android/bb/symbian) im client with very similar semantics to bbm. It's beta and noticeably buggy but functional.

[1] http://www.cnectd.com/