The biggest downsides to a laptop are cost of system, system support (replacement batteries, etc) and of course the overall system life.
Desktops are easier to work on most of the time and have a much longer system life because their design is superior. You just don't have as many cooling problems and the top-end processor speed is much better.
I have a Dell Lattitude C840 that's killer.....GeForce4 and awesome display....I actually play CounterStrike on it nightly. I would use my desktop if it weren't needing to be replaced. (weird rebooting problem right now)