Very addictive. What makes it so?
Some good points there philfrei. I think the quick-play nature of the game is important. You can easily slip in a couple of runs between tasks; you don't need to devote lots of time to it, as you would an RPG, RTS or similar. It's an ideal game for when you are procrastinating.
I think the game mechanics are also addictive. I can't quite describe it, but it confers a feeling of 'awesomeness'. For those 3 or so minutes that you are alive, you feel like the best 2D dogfighter on the planet.
The music really strikes a chord with me too. I like the slow 'chugging' sound, and it is good in its own right, however it reminds me of the Hell March theme from the original Red Alert (here if you haven't heard it: http://www.youtube.com/watch?v=Tb-gI_pFog0
). As such, I think it's digging in to some latent memories of my earlier gaming years. I don't know if this was intended, or an emergent effect, but it works for me.