Generally there aren't many games I instantly regret buying, since I'll probably play them for a good few hours anyway, but there are two that stand out that I got tired of within about 30 minutes. First was the Beyblade game for the PS1 which was easily the worst game I've ever played. Terrible, terrible graphics even for PS1 standard and the gameplay was equally bad, in that it was pretty much impossible to lose and you barely "played" anyway.
The other one would be the first Harvest Moon DS game. I mean, I loved Harvest Moon for the GBC and Friends of Mineral Town I still can play to this day. But the first DS game...wow. I mean, it was so bad. Had some stupid storyline that you had to go through and had far too many of those little fairy things, as well as horrible translation. I mean, the translation being bad in Harvest Moon is something I've learned to accept, but these were taking the piss.