youre gonna lose blackberry mail, browser and the pin messaging.
For browser just pick up opera mini which work quite well.
Youre gonna have to enter your internet apn setting in the option (just search the forum it easy to find (like on rogers for me it simply internet.com without password.
Further details go to this section of the faq How to use BlackBerry on Generic Data Plan - BlackBerryFAQ.com
for mms i dont know, i tried to use it one time (didnt know what i was really doing) and got an sms from rogers (my providers) about mms not implemented in my plan.