Biggest problem is mame doesn't have enough ram to load about 70% of the games in mame. Other xbox emus do a better job, but to give you and idea, newer neogeo titles and cps2 titles won't even load! So anything newer than 1993-94 many or many not run.
Other issues include that fact that you can't rotate games ect.... basically 50% of mame's options are no longer configurable. Not that this is a bad thing.... you just need to be aware.
Of course now the xbox is a mere 140 bucks, so There isn't any harm in playing around.