By far, this is one of the best games I've ever played, be it flash or commercial. I love it so much that this is actually the second time I've played it, the first being back when it first came out. The design of the levels, the unique appearance, the way the upgrades work, the storyline... everything is just so immersive and interesting. My biggest complaint is that it isn't longer, and it isn't a short game to begin with.
I hope very much that there is a sequel, or at least a spiritual sequel, in the works.