Wasup.
The Neo Geo roms that Mame-X CAN play, play flawlessly. No sound stuttering, no slowdown.(well, except for Metal Slug 2, but that has slowdown normally, so eh...) In fact, it looks better than my actual Neo Geo, because of the better television interface. It's second to xSNES9X as the most professional-feeling emulator out for Xbox right now. Its just a shame that it has a problem with larger romsets.
As for the CPS-2 games, I'll agree with Nerf on this one. I don't know what Final Burn does that Mame-X doesn't, but all the large Capcom roms that couldn't be played on Mame-X can and do play fine on Final Burn. Once you see Marvel vs. Capcom playing flawlessly, you'll be awfully tempted to invest in a couple X-Arcade joysticks. One caveat: Final Burn still has some problems with its control setup, so be prepared to spend some time fussing with configuring your buttons. That should hopefully be remedied in the next release however, so I'm picking nits on an otherwise great emulator.
Later,
mr jones