yes it completely varies from each game. Action games maybe don't work well at least you won't get alltime highscores; but rpgs and strategy games can work very well eg Advance Wars, Fire Emblem etc
i thought the controls being over the display screen in gpsPhone would be terrible but actually you dont notice after a bit
playing Fire Emblem as a gba emu tho makes me think how cool it would be if real companies were making real rpgs for the real touch with its touch interface... sigh...