Chatters of a feather flock together. That's the impetus behind BlackBerry Messenger, RIM's instant messenger for BlackBerry users that skips over SMS fees and goes straight to your pal's IM in-box. Version 5 takes a huge leap forward with a ton of new features: tiny thumbnail images of your contact so you don't forget who you're talking to, photo-sharing for files up to 6MB, and the ability to chat with groups of friends in addition to individuals.
Your method for getting those friends also gets a twist. You can use the person's unique BlackBerry PIN or their e-mail address to find them. If you're in the same physical space, one of you can also display your 2D "QR" barcode for the other to optically recognize through the BlackBerry's camera. Lest you fret that managing multiple open chat windows confuse you, you'll find that pressing the Menu key cues you up to switch among open conversations.
Our major complaint is that it's a beast of an app to download, especially if your data connection is weak. While using it over the data network is fine when you're not transferring files, you'd better be in a strong Wi-Fi area when initially downloading the app.