I wish
Most things I want to say have already been said in other reviews, so I won't repeat them here.
What I'd like to add is that I really wish there was a FUN "endless mode", instead of an endless mode that becomes too hard to win before you can buy all the upgrades you wanted, and that adds so many white blood cells and antibiotica to the screen that the game starts to lag...
At the very least, allow replaying earlier levels with your latest upgrades, to be able to actually ENJOY those upgrades! Buying an upgrade isn't enjoyable if the next level becomes impossible (in endless mode).