Connie and I played a game of d20 Yahtzee to see how it would go. It definitely lasts a lot longer than Yahtzee played with plain old d6s, and getting the special combinations (3/4 of a kind, full house, straights and crits) is a lot harder than it is in regular Yahtzee. Scores are a lot higher too. I got solidly trounced 1264 to 1023.
Connie came up with a few modifications for the special combos, like all primes, powers of 2 and other math-y related things. I think that calls for a new variant of the game.
I think there's something very satisfying about rolling d20s, although that may be a result of many mis-spent hours playing RPGs.