IMHO the emulators are perfect, both of them are plenty fast and full of options and useful tweaks, not to mention the magic on-screen controls
2 times out of 3 if the game doesn't work it's a crappy dumped rom... and the third left possibility for a non-working game is due to hi-demanding games...
Of course this is my personal experience that may vary from others, still usually when an application is popular it's mainly because it meets users' expectations... and all of fms work seems pretty good to me