This is a fun little time killer, but it is all about getting you to buy things in order to progress, and then if you do purchase energy you don't get the normal energy that refills over time. I bought 700 energy bolts or whatever you want to call them when I was at zero, I left the game with 450 left, when I came back to the game the next day I still had 450, the 52 that you normally gain over time didn't happen. Also the further you get the more energy it takes and you get less rewards.