This game is fun to start with, but quickly gets repetetive and tedious, and is also quite buggy. Here's a list of some of the worst issues.
1. Leaks memory like a sieve. If you leave it on high graphics mode, expect it to start locking up by level 40 or so. Even in low graphics mode, it's still slow as knobs. And speaking of:
2. If you switch it to low graphics mode, powerups no longer drop. While I'm sure this was unintentional, I found myself using this as a sort of "challenge mode"...which leads me to:
3. Ridiculously easy. Just make smart choices during the first 20 levels or so and for the next EIGHTY DAMN LEVELS you can basically just sit there. Which brings us to:
4. Endlessly, mind-numbingly tedious. 100 levels where NOTHING CHANGES. AT ALL. And after all those hours of tedium you are rewarded with a medal and a single paragraph of text. Yay.
5. For some reason it sometimes won't let you buy missile damage upgrades even when you have enough money. (It works eventually, so not a major bug.)
6. Medal for buying all weapon upgrades doesn't work.