Gut-wrenchingly, agonisingly hard.
Firstly - there's been a number of indie/flash games with similar mechanics to this, the most prominent being Clean Asia. It's not a common feature, but I've seen it before, mostly in bullet-hell shooters, so it's not an incredibly innovative idea, although it's pretty cool.
Now, I'm normally not adverse to difficulty, but this game is a whole new level of hard. You need to reset once every few minutes, and often the amount of points earned is disproportional to the actual skill you use in defeating the enemies. It takes forever to get to level three in all the upgrades, for example. That's probably because of the constant, unavoidable bullet hell. The game wouldn't be the same without it, of course, but perhaps slowing the bullets down or something would improve it, and allow players to earn more points in longer rounds, rather than being forced to click the reset button every ten seconds.
That said, it's a good showcase for a concept that needs to be used as a core gameplay mechanic more, and the upgrades were (mostly) genuinely useful.
8/10