I have found T-Mobile's support for Blackberrys is hit-or-miss as well. If you happen to get a tech that knows Blackberrys then they're aces. Too often, however, you get some mook who's reading from a basic script and has no clue about mobile technology in general, let alone Blackberrys. Here's my troubleshooting aganda:
1. Pull the battery to reset the Blackberry. This fixes more than half the problems users encounter, expecially generally slow/wonky performance. A subset of this is to delete and then resend your service books. Delete 'em, pull the battery for 30 seconds, then log onto your BIS and resend those service books. It never hurts so trying it is kind of a no-brainer.
2. Check the BlackberryForums (and for the love of God, use the search function!) for others having similar issues. Chances are you're not the first with whatever problem you're experiencing. These forums have bailed me out more times than I care to admit. And often it's something simple and basic, just a forgotten or omitted step. How do you think I got my Curve running on T-Mobile, browsers and all?
3. When you're at the end of the line, or if you suspect it's a provisioning error with your account, dial 611. And pray. Hard.