Oyshi definitely has one of the most expansive AYCE selections in Vegas. Lots of appetizers are also included, my favorite being the Surf and Turf. The fish is always very fresh, and their special rolls are inventive and delicious! There are a few reviews that specifically mention the Monster Roll. I like it, but I find it very difficult to eat due to the size. My favorites are the Sunrise Roll and the Hawaiian Roll.
I would avoid coming here during the lunch or dinner rush unless you're sitting at the counter, though...service tends to get really, really slow when they're busy and sometimes items from my order are forgotten. They do get very busy, so just keep that in mind.
Oyshi is by far the best sushi restaurant on the southwest side, and despite some slow table service during their busiest hours and some sloppy sushi-making when they are extremely busy during weekend dinners, I will definitely continue to frequent it when I'm in the mood for sushi.
Oh, and make sure to save room for dessert: the Green Tea Tiramisu is amazing!