I sold my MkIII a while back and have regretted it ever since, but I'm torn between replacing it with another MkIII or a MkII.
Ninsho is right about the new features making it more difficult to reassemble the gun. Don't get me wrong, it's not impossible, you just need to read the instructions carefully every time you reassemble (as opposed to putting together, say, a 1911, which anyone can do with their eyes closed).
On the other hand, the MkIII has a normal mag release (button by your thumb like any traditional western semi-auto). The MkII's release is on the mag well at the base of the grip. I don't like this for two reasons: one, I think it's hokey and goes against my natural instincts. But two, I've noticed that shooters (especially beginners) can loose muzzle discipline very easily when fumbling for the mag release. So for those two reasons, I'm leaning towards a MkIII.
Also, you can buy a MkIII new, if you're concerned about dealing with problems from a used gun.