This restaurant is one of our favorites, specifically for the buffet. HOWEVER, you actually have to ask for the buffet menu. It's a little sneaky, but if you know the secret, it's definitely worth it.
Is it the best sushi in Bangkok? I don't know. Is it one of the best deals in Bangkok? YES. The buffet is about 600 baht per person, and you can shovel all the food you can in 90 minutes, with complimentary green tea. If you want booze, you can either order the special buffet (200 baht extra), or by the glass/pitcher.
I'm not a sushi expert, but this is definitely better than certain other buffets with the conveyor belt system, because you can order what you want when you want it.
Our strategy has always been to order waaaay toooooo much for the first round and then sort of pick through the menu for individual delicacies. I'm not sure that's the way to go, but we always do it.
The hours here are weird, but if you're looking for a cheap, delicious sushi experience, I would suggest this one.