I had the 8125 and have moved back to an 8700 on Cingular. The form factor of the 8125 was a deal breaker for me. Dialing and inputing phone prompts on the screen was a nightmare. Also there is no one handed form factor to the 8125.
That being said, I actually love WM5 and will probably buy the GSM version of the Treo when it is available. the only thing keeping me from firing back up my Treo 650 is that I travle internationally and BB is the only phone that offers an unlimited international data plan.
My 8700 does everything I need it to do. If you want a lot of multimedia stuff then a Treo or 8125 like device might be good. If you are a business user who needs wireless access to your contacts, e-mail tasks, memos and calendar, the 8700 is rock solid!