> Interesting...Gotta try it
> How does performance compare to MAME (n00b question, I know)

JEmu2 is java based, so expect MAME to be faster. I just compared with 2 games and they were 18% resp. 28% faster in MAME.
If you can get Java Web Start to use the 'server VM', you will get better performance. With those 2 games it was as fast as MAME for the 1st game, for the other game MAME was about 17% faster.

Emulator development involves a lot of things where java is not really good at, performance wise (array access everywhere).

EDIT: I tested using the 1.6 client VM (early access snapshot release) and the 1.4.2_03 server VM. Tested against MAME 0.37 (really old version I know, maybe the latest MAME does better but this version is the only one I have lying around right now. TBH I expected the difference between MAME and JEmu2 to be much bigger).