Yup, it worked pretty well on a store-bought 3*3*3 =)
But it didnt seem to have much effect on Rubiks 4*4*4, I'm guessing because it's too loose already...
Anyway, for a gigaminx, since it's probably too loose for sand to have too much of an effect, what I recommend is, perhaps you could figure out what's causing all the lock-ups in it, then get sandpaper/dremel/knife/whatever and cut down on whatever's causing all the lock-ups. Though such a chore is not for lazy and impatient people, because if you assume that every piece needs to be sanded down at specific areas, you have 242 pieces to sand.
But it's probably worth the effort! At least for my rubiks 4*4*4 that is, because my average went down by about 15+ seconds immediately =)
Oh, and speaking of gigaminx, I ordered a white DIY from cubeforyou, and they gave me a BLACK, ASSEMBLED, STICKERED one... and OMG, they used PP stickers, and they're peeling already... So the only 2 things they got right about my order was my address and the fact that I ordered a gigaminx...