Classic, and way underrated
I found the game mecanics fantasic... I always sucked at it, but it's still fun. I was still able to make it to the end, and beat the mother ship. Love the Congradulations screen. :P Everything looked great, though they were sprites, they were still awesome. While some of the hand-drawn stuff seemed out of place (namely the pylon), it all pulled together and was a really enjoyable experience.
It's a shame that it appears all the smart NewGrounders are doing easter things today and all the retards were left to review this game. My only suggestion would be some ships move to fast, you said that some move the same speed of the cannon, but if thats true you can't catch up with them, although I see how that can work if you're on the left side. Maybe those should only be in the later levels, adding a different kind of difficulty other than needing a higher score.
But my complaints would definatly have to be that space invaders are WAY to weak, and the numbers are totally broken. Hope that my not-elaborating helps you fix your game. -.-