I liked the premise, but...
I DIDN'T like the way that you were unable to recover health at all, ever. The fact that you had utterly finite health meant that a single mistake could cost you the entire game, but you wouldn't even know it yet. You could be like me and die right at the end of the final battle (in the normal missions), simply because you didn't have enougth health left to take that final hit, and all because of a mistake you made back when you were still learning the ropes 10 levels ago. (Or worse in my case because freaking Microsoft decided that was the right time to pop up on my screen with a message about some update I didn't want, and it got me a fair bit of damage)
I could have forgiven a system like that if it maybe saved my progress, but it made me restart the whole 15 levels from the beginning, and I had been just moments away from winning the final level. Not cool.
The challenges are kind of meh. To be honest, I feel the game is far too simplistic for all these complex awards and modes. Let's face it, it's basically asteroids with really annoying ammo instead of really annoying controls. You can't make a whole series of interesting challenges out of that, because once people get the gameplay down-pat they've really seen all the game has to offer. Sure, you can increase the difficulty, but it's ultimately the same very simple combat just repeated in a different way.
I gave this a 7/10 because I did enjoy the gameplay mostly, and I did like being able to upgrade my UFO, and simply because you put so much into the game. That said, I would have preferred to see a much faster rotation speed (it was almost pointless since the damn ufo couldn't turn fast enough to shoot in the direction I wanted before I was overrun by dot things or worse, an annoying enemy UFO with super speed), perhaps more varied upgrades (since stuff like the glueton bomb tended to actually be bad for you if you were dumb enough to use it), and a less unforgiving level/health system.
If you'd had those then I would have appreciated the game much more.