Fun, but feed your pets first.
Fun game, good design and sound though the upgrades could be a bit more robust. The cats and dogs were an absolute terror and served to ramp up the difficulty, thus the warning. Feed your pets, make sure they have water and know you love them because after the 20th level or so you will need some time to not want to instinctively harm anything that meows or barks.
There are also a few dead spaces, such as the left side of the buildings at the bottom of the screen. If you kill enough to get the most powerful gun and the most powerful dog its possible to create a crossfire there that nothing stands a chance of getting through to hurt you.