Fun
This is a fun game - especially because it harps back to space invaders. I found it reasonably difficult - I restarted twice. Although I thought the last boss was easier than the first. The graphics were pretty and had a good and appropiate feel to them.
I liked the fact that you could upgrade but what I really liked was the way inwhich you got the bonuses. From what I could tell - shooting down the bonus part of the ships could make all the difference to whether you were able to fully upgrade or just partly upgrade - this adds an element of replayabilty and challenge which is just swell.
I felt the game needed some kind of ending - even if it was only a page saying "The End". There were some minor glitches with collision and occasionally when the ships changed position rapidly there would be nothing you could do but get hit if you were standing in the wrong place at the wrong time.
Generally a fun game that harps back to simpler times and... complicates them :)