We eat out and travel a lot. Vienna is a wonderful city - but .. Asian food is far from its strength. Sashimi especially is a tough ask in this city. At last count 10 different 'sushi' places and I would not go back to a single one. On a tip from a local friend we visited Mochi (2nd time now) - we were open to the menu. This is not a straight sushi bar, this is fusion. That being said - we are sashimi people. I never order anything on the menu until I taste this one dish and here it was excellent. Easily the best we have had in Vienna. It was, melt in your mouth good. Served at the right temperature and the correct thickness of cut. The edamame was also good and fresh, served with fresh shredded horseradish. I can see how 'die hard' sushi lovers who want their version of 'traditional' Japanese food might struggle with the menu. But keep an open mind - try some dishes and you will enjoy the dining experience.
On the afternoon we were there, outside was fully booked - back to back. This place is TINY ... so booking or at least calling ahead is necessary.
I agree, the staff can be a little slow - but all in all they did a good job and spoke very good English. This tends to be more of a local place - so be prepared to dine with people next to you that may have their dog with them (like us)
As with all reviews this is a personal opinion - but we have had 2 visits here and both have not disappointed. It is sad when people can't place balanced reviews. We will be back again - but next time I will book ahead.