I have a blackberry pearl with t-mobile, and it's not bad. The data rate is slow (200k if you're lucky, usually much lower than that), but it lets you use the DUN bluetooth profile when connected with OSX, so it's an acceptable wireless modem, and a decent phone. The DUN profile only seems to work with my powerbook though; I've tried using the phone as a bluetooth modem with my nokia 770, and the blackberry won't adv…
non blackberry devices get unlimited data for $20, this is why I'm trying to avoid blackberry devices, or at least avoid letting them know I have one.
Unless you're grandfathered into the ancient $5.99 T-Zones plan, I don't think you will find a cheaper option.
Also, I think a BlackBerry without the BlackBerry plan will be heavily neutered. I'm guessing email won't work because it depends on the BIS backend. BIS is used for "personal" email things like your school's Exchange server, your Gmail account, and your IMAP/POP account. BES is used for hooking up with corporate Exchange setups running BlackBerry's Enterprise backends.