Well, I got my hands on a T-Mobile MDA today and I'm impressed with the size and the feel of the phone. I've got a T-Mobile Blackberry 7100t (note that the link takes you to a 7105t which has superceded my model). The MDA is a little shorter and a little thicker than my phone (the custom-fit case that I have will just barely accomodate the MDA's thickness) which means that the form factor is just about perfect. Since the keyboard slides out of the MDA, the screen is much larger than the Blackberry.
Have I decided to buy one yet? No, not yet. The slide-out keyboard seems a little flimsy and it's a little dated. I need to do a little more research and then make my decision.