Many Japanese restaurants looking to bring in business offer "All-You-Can-Eat" (AYCE) sushi menus, some only on specific days or nights, others every night. While the "All-You-Can-Eat" gimmick certainly appeals to many American diners' preference for quantity and large portions, does the "quality" measure up to regular sushi offerings or not? In general, I'd have to say no, it doesn't.
AYCE sushi can easily suffer from many ways that restaurants can look to reduce costs while keeping diners' bellies full. They may pack the nigiri sushi with greater amounts of rice to fill up customers without having to use too much expensive fish. The fish may not be the freshest - be wary of restaurants that, perhaps, only offer AYCE sushi on one day of the week - that's probably the day they DON'T get a fresh shipment or are trying to use up old inventory.
AYCE sushi restaurants can also pull other tricks to minimize their cost, such as limiting selections to less expensive items such as crab, egg, octopus, etc and charging additionally for premium items. They may penalize you for any pieces of sushi not completely consumed by charging you full price for them to discourage over-ordering. They may also try to fill you up with "free" soup or salad before you sushi comes, and bring out your sushi very slowly, to discourage you from ordering more.
This is not to say every All-You-Can-Eat sushi meal I've experienced has been a bad one. But I would say that it is very, very rare and unlikely you will find the best sushi in your town or area at an AYCE sushi restaurant. The best sushi demands a premium because of the quality of both the sushi chef's skill and the quality of the fish they are serving you.