The alpha channel is not killed when you render with motion blur, but it doesn't exactly behave appropriately. The bluring will push out the mask so you will end up with a line around your figure when it is composited onto a background. Obviously this line gets thicker in areas where there is the most movement and more blur. If you can select a background color in Poser that matches the background your figure will be composited against, this effect can be minimized.

The rendering time seems to be very quick, but since it's rendered with everything else it's hard to tell how much time the MB actually takes. In a simple scene with one character in the default lighting setup it only appeared to add 1 or 2 seconds to each frame. It's just a checkbox available when you Make Movie so there is really no control over any aspects of it, but I've been very happy with the results I've had with it so far. I just got After Effects though, and don't have any other programs with MB, so I really haven't had a whole lot of time to compare and contrast.