Not all that great.
Like I said, It's a very basic program: you watch your virtual fish as they swim around their pretty tank, and fish acknowledge your existence as you move your hands. They scurry when you move fast, and you can make the water move like you're dipping your fingers into the tank. And really, that's it. I suppose I was hoping for more interactions - feeding would have been great, and at least something more than just staring at them. A counterpoint to that would be that it's fish - how interactive can they be? So I suppose I could have known what I was getting into.
That being said, it is very pretty to look at, and makes for a fantastic screen saver (the larger your TV screen, the better). But even though it's dirt cheap, I don't think it's worth buying at all.