For Japanese food I mostly visit the cheapo places like Yatai (See http://chanchao.fotopages.com/?entry=75377(review)) but sometimes a relative splurge is nice too, and the food and surroundings definitely a step up. For starters it's nice to see seriously potent wasabi readily dissolving in your soi sauce, rather than see it and up as little chunky lumps floating around. :) And the surroundings make a difference too, a nice air-conditioned room or outdoors riverside dining area with plenty of staff is definitely nice for special occasions.
Kitamon is perhaps not the total high-end for Japanese food in Thailand, prices are still quite reasonable. It's a bit in the same league as the Japanese restaurant at Nang Nual on the other side of the river. Kitamon might just have the edge in this range, though Nang Nual does an all-you-can-eat buffet both lunch time as well as dinner time. Kitamon has this only at lunch time. Kitamon also does home delivery, and prices really are no higher than your average Pizza Hut delivery..
There are separate menus for Japanese food as well as Chinese/Thai food. The Chinese menu looks very interesting as will, with some good seafood choices. Will especially try the Crab with black pepper sometime. On this occasion though I stuck to the Japanese food which I think turned out very good indeed. There are some 'sets' on the menu, including sets with Japanese rice and grilled fish or meat, or sets with various sushi or sashimi items. The biggest set goes for 400 baht, the one pictured was the smaller set for 300 baht. It's served in an interesting little boat-shaped tray filled with ice to keep things cool. The nice presentation and friendly and fast service make this still very good value for money I think.
Kitamon is on the river, on the Western bank, South of town before you get to the Aom Muang ring road. It's on the left side of the road when coming from town driving South.