Similar to Keena E., I've been on the quest for the best AYCE sushi in Las Vegas as well. I've been to Sushi Koma 5 times. And I've waited until now because it's a small spot and I don't want it to get busier ;)
I've been to lots of other great places - including Umiya, Jjanga, Goyemon, O-my, and Toro. Sushi Koma is still top of the list (with Goyemon as a close second, and Umiya as a close 3rd).
Their fish to rice ratio is amazing. The seared tuna carpaccio and seafood tostada are insanely good. They've never been out of Uni (unlike Goyemon, which didn't have it). And their servers are insanely fast.