I mix pills once in awhile and the only difference I've noticed is that sometimes I'll get a better roll. My guess is because of the lack of cross-tolerance. For example if you have a tolerance for MDMA, and you take 2 MDMA pills you won't get as good of a roll as if you took an MDMA and an MDA pill. But when you do mix, you build tolerance to both types as oppsed to just one, so it has it's good and bad points. The ideal would be to know what exactly is in a pill, but of course that's not an easy thing to know. OK, that's just my .02
peace,
Wyrm