Not a bad time-waster, but I ran into way too many bugs to get a good impression of the game. For starters, as I played through the game, graphical glitches started developing. This in an of itself wouldn't have been too terrible, but at around wave 13 or 14 the glitch became a large horizontal glitch across 95% of the length of the game field. Again, this wasn't too terrible, except it kind of got in the way of flying enemies. The problem was that it acted as a barrier for my grenades and missiles, causing them to detonate well before they hit enemies or the ground. Add to that the growing slowdown that each wave caused (perhaps due to the graphical glitching, I'm not sure), and the game grew more and more troublesome the longer I played. I decided, once it got unplayable, to reload the game, at which point I found perhaps the worst glitch: the fact that any upgrades do not apply to a loaded game. Unlocked weapons will still be available, but at their default strength, fire rate, and clip size. I'm not positive, but I believe the powerups were also reduced to base levels as well. This was a big problem for me, since I reloaded the game after I beat wave 19, meaning that I loaded into the boss wave with default weapons and powerups. I couldn't possibly beat the boss with the default pistol, so I had to use other weapons, meaning that I missed out on the "use only pistol" medal I had tried hard to collect throughout the course of the game. Then again, I didn't realize the boss had weak spots, so perhaps I could have still beat him even with the crappy pistol, but I digress. You can see how this problem would make Hard Mode pretty much unplayable upon continuing; the player would be required to play all 20 waves in one sitting in order to avoid the glitch.
The weapon upgrade glitch I mentioned above is the only one that's easily repeatable; simply close the game and load a save file, and all upgrades will be lost until you clear the current round and move on to the next, at which point any and all purchased upgrades will apply again. I can deal with the graphical glitches, as long as that damned save glitch is fixed. Until then, I can't rate higher than 2.5 stars.