We redeemed our Fatburger Groupon the other night. It came with two cheddar Fatburgers and skinny fries, and we ordered milkshakes as well (Oreo for me, strawberry for Scott). The burgers were good, decent meaty patty with lots of toppings. Not too greasy or anything. The fries were pretty standard. The milkshakes were nice and creamy and smooth but pretty expensive for the size (compared to the last burger and fries and shakes place I went in Calgary).
Fatburger has some bad reviews on their Urbanspoon page but it seems like they are for the more speciality items on the menu. Their classic burger seemed perfectly fine to me.