You know what I don't like about MAME? 1 word: ACCURACY. I emulate a lot of side-scrolling and vertically-scrolling Shoot-em-Ups in MAME(and some 3D rail shooters), and I've compared MAME to the original arcade boards and realized I'm not getting the proper experience with MAME. With the games I emulate, it's so horribly off it makes me sick. Konami games are the most affected, and I will list some of them:

-Gradius/Nemesis: In the newest releases of MAME, the colors are WAY too bright, and in all releases, the volume balance between the different sound chips is wrong, the music plays too fast and generally sounds WAY off compared to the original board.
-Salamander/Life Force: Volume balance is, again, quite a bit off, the Stereo sound is reversed, the music plays WAY too slowly and any sound produced by the emulated VLM5030 is almost completely wrong
-Gradius II/Vulcan Venture: Volume balance problems, music is too slow, START screen seems to disappear too soon, emulated uPD7759 seems off
-Gradius IV: Severe performance issues, music is too slow
-Salamander 2: NUMEROUS graphcial errors, recent versions of MAME have major speed issues with this game, sound effects are missing extra echo/reverb effects
-Super Contra: Incorrect volume balance, music is considerably slower than on the original board
-TwinBee: MAJOR volume balance issues, missing sound filters

Other problem games:

-After Burner II: Emulated YM2151 produces incorrect notes
-1942: Sounds off
-Arkanoid: Has issues since the AY-3-8910/YM2149 core was updated
-Thunder Force AC: Flaws with the emulated uPD7759 and YM3438, noticeable volume balance issues, music is a bit too fast
-Raiden and Zero Wing: Emulated YM3812 leaves a bit to be desired

I've actually reported many of the issues with the Konami games listed to MAME Testers, and what have they done about them? ZERO! Absolutely nothing. So the hell with that, for use on a modern Windows XP/Vista/7 computer, I compiled my own modified version of MAMEUI v0.138u1 and will not upgrade to any newer versions of MAME until the MAME team improves on the games I like which are far from perfect.

Along with that, I've got an older computer running MAME for MS-DOS v0.36 and v0.57, both versions which I use exclusively for games using a Yamaha YM3812 for sound. If you recall, versions of MAME prior to v0.60 can make use of Yamaha YM3812s and Yamaha YMF262s on older ISA sound cards(the early SoundBlaster sound cards come to mind) for 100% authentic sound off those games. With that in mind, I set up the computer with a SoundBlaster Pro 2.0 sound card(contains a YMF262, which is backwards-compatible with the YM3812) and have set up both versions of MAME to use the YMF262 on the SoundBlaster Pro 2.0 rather than MAME's emulated YM3812/YMF262 depending on the game. And I will never upgrade any version of MAME until the MAME team straightens things out with the problem games I listed.