Overall, this is a fantastic game. I do however, have a few gripes.
First, the "Kill it with Fire" achievement doesn't seem to count flamethrower kills against the Crawlers in Eternity Endless mode. I'm not sure if that's intentional or not, but it makes getting that achievement incredibly annoying.
Second, in the Eternity Endless mode the weapons displayed in the shops don't match their descriptions (the descriptions reflect the true weapon).
Third, in the Eternity Endless mode the cost of an item in the shop is sometimes determined by the cost of the item that replaces the item you purchased, which occasionally results in paying more for something than you would otherwise and can actually result in negative quantities of the shop currency.
Fourth, the game tends to crash during the Eternity Endless mode due to all the effects happening on the screen once you've maxed out all the possible perks (Which should happen around level 150). The only real way to stop this is to be very judicial with your firing so as to trigger fewer effects (made it all the way to level 250 before crashing using this method). So, that kinda sucks for those of us who might like to see exactly how far we could make in Eternity Endless mode (I was hoping to reach level 1,000).
Now, while I'm not going to go into the merits of this game, don't take that to mean that there aren't any. On the contrary, in my opinion, everything about the game is perfect, except for the few things I mentioned above.