As a tapas bar, it's fine. The volume of food is pretty light, and the service seemed a little slow, but the size and makeup of our group clearly overwhelmed them. We definitely came out ahead of what we paid. What bothered me was when I called they had no record of our Open Table reservation. Fortunately they were able to accommodate us, but I shudder to think what would have happened if I hadn't called. So yes, you can use them, but you'll need a Japanese speaking person to call because the Open Table reservation doesn't do anything. I asked, and they explained that Open Table reservations go to their main office, who doesn't forward them to the actual restaurant. That's a big problem, not with the restaurant, but with the restaurant's integration with Open Table.
OpenTable ダイナー2名によるUMIバル 新宿店の評価は3.7☆です。