In the interest of fairness, I gave this place three opportunities before writing a review. All three visits produced similar experiences.
First, I will say that this place is very clean. I imagine it's fairly easy to maintain since the customers are so few and far between. 2 of the 3 visits were during lunch, and one for dinner. I was the only customer for both lunch visits and one of three for dinner.
The service is polite but incredibly slow. I was literally the ONLY customer in the place for two of my visits, and it still took 10 - 15 minutes for them to make a single sushi roll. I ended up leaving not because I was satisfied, but because I was so tired of sitting there after 90 minutes and only 4 orders delivered.
The worst part was the food. The rice was cold and horribly soggy. It was sincerely disgusting. The nigiri was a huge ball of nasty, soggy rice with a tiny dry piece of fish. The rolls were about average size, but the quality of the ingredients was terrible and almost inedible. Just the thought of that rice makes me cringe.
I have eaten at most of the sushi places in Las Vegas, and Zen is by far the worst.