depends, Naz is the expert, but in my opinion, they are not as good as the correct sized toothed v-profile belt, however, the fact that you can change the length on the fly is nice. there have been a few instances where i would have been stranded if i hadn't been able to adjust the belt length. you should see naz, he always has a few extra belts of different sizes hanging off his bars. for the high rev 50cc bikes, the powertwist just slips too much.
as to gates or malossi, i just use any old generic with a nice thick profile. here is a link to the source of my last batch: