Mine have plenty of fish in their diet from other food, so we don't supplement, but for cats that don't it should be fine. Tuna in particular is one of the fish you don't want to feed too much of though, but cats sure to looooove the taste!
As others have said, its not really "cat food" its just canned tuna, so not really a balanced diet just a supplement.

The price is very low though. The best deals on human-grade tuna I've ever seen were around 55cents for 6oz worth. If that was a clearance price (that's how I got usual $1 a can food at 50cents a while back at HEB), I wouldn't be worried, but otherwise that is pretty inexpensive and makes you go "hmmm" as usually cat food manufacturers have to put in at least SOME inexpensive fillers like rice and the like to get the price down that much.