It's okay.
Not incredibly pleasing aesthetically, but that's half the point. Too much detail in the visuals and you can't pump out the processor power to generate all those cells.
The main flaw in this game, as others have stated, is the repetition. It's fun for a few levels, picking off every single one of the cells and feeling like some kind of super-parasite, experimenting with the upgrades to find they make so little difference without dropping 10 sets in it that it's not really even worth the effort to TRY and reach a state of complete dominance in the game.
The only seemingly useful upgrade I could find was the bomb refire, since it was the only thing I ever really used. Your own size increases in such small increments that you don't even notice the difference, and your buddy seems to be suffering from some sort of paralysis, so why even bother with them?
The game doesn't need to be challenging. I would have been perfectly content if I could constantly spam bombs every half second by the 8th upgrade. I would have called it too easy, but I would have given it a higher score - I guess I'm just fickle like that.