There's plenty of BlackBerry Twitter apps around, but whether you can get any given one of them to work on any given handset is something of a crap shoot. From that point of view, RIM's official BlackBerry Twitter app, currently in beta and expected to be formally released in the first half of this year, is a welcome development.
The beta software seems very preliminary — RIM invited me to try it out but so far it hasn't managed to get past the sign-in screen. There'll be a full review if it ever decides to start working properly. In the meantime, you can sign up for notification when the official release happens.