Aji Sai is a good choice downtown for all you can eat sushi. There is a lot of selection on the all you can eat menu - even for those who don't like raw fish. The sushi is what you expect from all you can eat - a bit scaled down versions. The service is so/so. The price is as expected (around $20/person).
Make sure you try the cream cheese won tons and the seaweed salad.