I've enjoyed this game a great deal. Unlike many, I didn't find the number of threats impossible - the game is unforgiving but fair. Personally, the unlockable environments didn't add much value for me - I'd be much more interested in bonus levels that give you a certain enemy composition. All purple or all green challenges, for example.
But honestly, I'm reviewing for one reason: would you PLEASE tell me what the "End of the Line" medal is? After playing for a while, I have to admit that I'm probably not going to reach it. I finally survived more than five minutes today and didn't see anything new, so I surrender. Would you please sate my curiosity?