I've been using an MDA since Feb. and I had to have the Pearl and so far I am loving it. But the video compression on the MDA was so much better and not just because of the screen size. Still pretty cool to be able to fit four or five full length movies on that little thing.
You can use a program called SUPER to convert your .avi, .wmv, .mov or almost any format into .avi for the Pearl. There's more info at BlackBerry forums. Then you put them on the device w/ BT or USB. They won't play on my PC though so I can't test them there first, only on the Pearl.