I've been to Yellowtail a few times, including twice in the past month. My experience has been great every time. Both times I recently went we sat right next to a window overlooking the Bellagio fountains and the food and service were both phenomenal.
I've had the edamame, big eye tuna pizza, the Alaskan king crab tempura, the popping spicy crab aka pop rocks roll, and the filet mignon toban.
The sweet chili that was added to the edamame made it very delicious. The big eye tuna pizza is a dish that is an absolute must try. It's difficult to describe the burst of flavor when you bite into it a piece, but it's excellent. The crab tempura was fried to perfection and went really well with the sauces they served it with. The spicy crab roll was unique because of the pop rocks they add to it. The filet mignon melted in my mouth. I also had a dessert that was made from nutella that was amazing. I don't remember what it was called (dessert in a cup maybe?), but I would recommend it as well. The overall quality of the food is good enough to justify the price you pay for it.
I would definitely call in advance and make a reservation. Also, a lot of people dress up to eat here, but the atmosphere and vibe is laid back and relaxed enough that you can wear relatively casual attire and not feel under-dressed.