Challenging, but fun
At first I rated this game a four, but because after playing the first time I couldn't help coming back again and again in an attempt to finish it, I must say that I am addicted and I therefore believe it deserves a five.
Yes, of course it has it's flaws, but it's genuinely different and fun, and it's a good challenge!
(I'm currently stuck on the last campaign, but am looking forward to finishing it at some point - it feels as if there are specific configurations of guns you need to get through the campaigns. One complaint I'd have is I can't see ways to make guns like the apocalypse cannon useful, as they get destroyed so quickly! Though then again, I haven't made too much use of thesecondary turrets.)