Zushi is my current go-to all you can eat sushi.
There's plenty of other locations downtown, but Zushi has been so far the most consistent in terms of food quality. As it is with many other sushi joints in Montreal, the owners are Chinese, and so are most of the staff. The menu is pretty similar accross the board, so I guess the main reason for my appreciation so far has been the lack of unpleasant surprises. Don't get me wrong, I think the stuff is great, but before raising your gourmet expectations, remember that Zushi is the fast food version of sushi.
Everytime I go, there's no or little variation in my orders (with all you can eat restaurants, I prefer to save my precious stomach space for food I love most).
I start with miso soup and seaweed salad, a LOT of edamame, then tempura shrimps, popcorn chicken, sashimi, a whole bunch of sushi rolls, and fried rice if I still feel a bit peckish. A few of the hot dishes are pretty good too, but I don't recommend experimenting with many at a time. I also tend to avoid handrolls, nigiri, and the ever-perplexing pizza sushi. I always finish with ice cream (the green tea flavor is nice, unlike the shampoo-tasting version from Seoul Chako *shudder*). You do have to finish the food to avoid paying extra, but I found that a few leftover noodles or half a piece of chicken never got me into trouble.
There has been up and downs with customer service, but it's overall fast and pleasant. If you're not sure about the whereabout of an order, just enquire about it. They only accept cash, which is pretty annoying, but at least there's an ATM machine next to the door (extra fees meh). Pricing is fair, especially at lunch (but sadly no sashimi).
Bonus: There's two Pokestops nearby!