I tried the Kozo Sushi outlet at Thaniya Road (inside Thaniya Plaza, ground floor) today, and agree that it's very good. They didn't have an all-you-can eat buffet going on at the time (it was either the wrong day or wrong time), but it was good and cheap nonetheless.

You can either order off the menu or take what you want from the conveyor belt. Dishes are priced from 10 to 50 baht, and all the ones I tasted were good. My friend and I feasted like ravening hogs and the bill came to only 570 baht.