Absolutely loved it! Though I had some minor issues playing the game in the safari browser, as the game's level would "finish" while some of my shots were still being fired.
I second HookahBird's thoughts on a sequel! I do think that the "pigs in space" idea has some serious potential. Maybe they're trying to defend the solar system from an invading alien force? That would give you some very fun flexibility in terms of the environments, and some of the "space battles" could have some interesting physics around asteroids super mario galaxy-style. Or just no gravity at all!
Some possible weapons here would be pig UFOs, a black hole gun (maybe sucks up the bullets and missiles and pigs into a single point so they all go boom?) and maybe an asteroid caller for some nice explosions.
Another alternative to that (though probably not as entertaining) would be "fighting through time," where you could have them fighting dinosaurs, cowboys, knights, etc
I always have tons of zany ideas that could probably work well for this, can send me a PM if you like. In the meantime, keep on making these awesome games!