I've been to Nippon on various occasions with various types of people. The atmosphere is unpretentious and laid-back. Sushi is average, never bad. Foot traffic is moderate (surrounded by restaurants and the Museum District) and customers range from older gray-haired couples to Montrose crowds, so people watching is great. HOWEVER, although the place is staffed with at least 2-3 people at a time, service can be INCREDIBLY slow. When I go with a group of girlfriends, I always get great service. But if I'm there with only one other person, it could take up to 20 min after being seated for someone ask for drink orders / give menus and up to 45 min!!! for the sushi to be brought to us. On one occasion, my male friend actually had to go up to the sushi bar to ask for our food. Albeit, if we express concern the waiters always apologize but I haven't seen a change in service. There are definitely better, cheaper, faster places around Houston.
Pros: Simple Ambience, Good Sushi, Location, People Watching
Cons: Service is Horrible (esp. empty weekday afternoons), Parking can be Difficult