Is it the FW800 or FW400 MDD? Only Pro if it's the FW400 one is that it can still boot to OS9 if you need need certain apps. There are plenty of cons though: it's a very loud machine, won't run Snow Leopard but will run Leopard quite nicely for the next few years. Come on people still use Panther and Tiger. I don't think Leopard will be obsolete any time soon . DDR is still expensive too but you can get 512mb modules for pretty cheap though.
It's still a good machine otherwise for basic stuff.
you can't run Imovie '09. I finally moved off of my MDD for that (and other reasons). It is slow with iPhoto with lots of images 30,000+. Other than that... great machine.