A word here about a shortcoming that I've bemoaned in Windows Mobile before, and will again: It doesn't clean up after itself. You have to shut down applications, usually by tapping an OK button in the upper right of the display. Sometimes there's an icon at the bottom that allows you to exit by pushing the hardware button underneath; most phones have these left and right buttons for navigation. But otherwise you need a stylus or a fingernail.
The Treo 700w has a dedicated OK hardware button to take care of this problem; the MDA doesn't, although you can reprogram one of its function buttons to do this if you wish. Similarly, unless you slide out the keyboard, you cannot launch the Start menu without tapping the screen; the Treo 700w has a button for this, too.
Am I being too nitpicky here? Maybe. But T-Mobile has lofty ambitions for the MDA, and it fulfills them only up to a point. I'd say the MDA is worth a look if you're looking for a tiny laptop substitute and aren't planning on heavy-duty phone use.