Ginza Hohsen reportedly has gems in its menu, but it's honestly too expensive for us to find out. If you're looking for good sushi, sashimi, katsu don and Japanese food in general, there are far better choices in Colombo (Nihon Bashi at the high end, Sakura at the low).
We tried the basic sushi and katsu don and, while fine, nothing blew our socks off. Except the bill, which was nearly Rs. 6,000 for two, and not for a very filling meal. We each had about four pieces of sushi and half a bowl of katsu don, essentially sharing food. If you were to get a range of things you liked the damage would exceed Rs. 10,000 for two, and it's not worth it.
Perhaps if Colombo had a dearth of options for japanese food, but we don't. Seriously, Nihon Bashi near Galle Face Hotel is much much better, and actually cheaper. But enough of that. To the general ratings.
Ginza Hohsen is in the Colombo Hilton. As you enter the lobby turn left, then go right at the end. The Hilton staff are excellent and their restaurants and pubs produce consistently high quality fare, though it is rarely awesome.
Ginza Hohsen itself is a bit of a strange combination between a diner and a Japanese restaurant. Nothing stands out in terms of decor except the buttons they have on the tables (waiter, bill, water). Those are pretty great and save a fair amount of time and hand waving.
The Ginza Hohsen quality is good, it's just low value for money. The sushi was OK, they seem to consistently have tuna (places seem to run out) and their katsu don was flavorful and really quite good. It's just that in the scope of Colombo you can A) get higher quality Japanese for cheaper or B) get much more quantity of Japanese for cheaper. Ginza Hohsen gets you the disappointing experience of paying a lot for not much.
The service is excellent, as it usually is at the Hilton. The buttons on the tables are an especially nice touch.
Ginza Hohsen is fine, but we couldn't recommend it unless it was the only Japanese restaurant in Colombo. And it's not.