i had a look a few hours ago. things like genesis look spot on. and in fact, since i have a PAL genesis and the GP2X gen emu automatically selects 60hz it looks better than my original machine on the same tv!
my version of mame is 3.1 (i think) and it has a couple of issues on the tv that dont concern me. i actually bought my gp2x to run a telly in vertical mode, which it does well. in horizontal mode, i notice that some games have a little bit of the top chopped off. i tried robotron and it did this. yet other games, like metal slug 2 are perfect. if you arent in a rush, i might try the latest mame which is v5.1 and see if theyve resolved that issue. so in a nutshell, all vertical games that are run horizontally, and some horizontal games that are run horizontally have a little bit of the top or bottom cut off. like i say, this doesnt bother me because i specifically bought this to run vertical games on a vertical monitor.
the quality of the picture, as far as i can tell is about what you expect from a pc with tv out. again, some of the older games have a bit of bleed (robotron) but newer games like metal slug seem quite clear. other people will be able to tell you why. i think that is a tv out issue in general though and you might want to look at some of the tv versus arcade monitor threads in the monitor section. there might be some pics to compare. i tried to take pics but they really didnt work very well (and yes, i switched the flash off).