Ditto on the "spin-down" D20s (which are actually life-counters for MtG, if I remember correctly, not dice per-se)
I have played with people using marbled dice with difficult to read numbers that infuriate me... almost to the point where I wish I could buy them a set of flat, ivory colored dice with black numbers and force them to use it.
However, I know my players well enough and I know they would not cheat...
And, really... Anyone contemplating cheating in a roleplaying game is a sad, pathetic little person.