The best Twitter client for _any_ platform, BlackBerries included, is your XMPP-based instant messenger program. This is because IM (and SMS, but we'll ignore that since you're on a real phone) is the only way to access the Twitter's 'track' feature. Their web interface, and their API interface, only supports 'follows'.

So fire up JiveTalk or RIM's Google Talk client and have at it.