It is good, BUT...
The normal mode is easy enough to play, however when you go through all of the levels the only levels you can play are the endless versions, which are LITERALLY IMPOSSIBLE TO PLAY. You start off with a minimal amount of units and are forced to go up against about 50 enemies at a time, and if you actually somehow manage to survive that one round against all those enemies, the upgrades do NOTHING TO HELP YOU. I seriously ask you to either lower the difficulty of endless mode or at least allow people to do the normal version as a separate game mode, otherwise the game will just stay rigged.