The best way to spend the money is getting the next tier unit. Dont upgrade. Next unit is normally way superior. Especially the penguin, which will just freeze and kill from a far. Also wear the EXP and Money rings, so you dont have to go back to grind.
I love it. 5/5
Instead of doing a grinding game, there is actually strategy behind the tiles to uncover, the experience points to spend, the upgrades to buy and the items. One complaint is that some permanent upgrades seem awful for the price.