Pretty good, would have gotten a 4.5 if I hadn't gotten to the last level, as up to that point, the only real problem was not being able to redo the path easily.
However, that last level is awful. If the endermen went to every single corner, it'd be ok. If endermen didn't occasionally decide not to take damage from water, it would be ok. If when the dragon destroyed your towers, the inflated cost went back down, it'd be ok. If screen transitions caused the inflated cost to go back down, it'd be ok. If endermen didn't sometimes decide to just teleport to the end of the path, it'd be ok. If there was a way to refill your health, it'd be ok. If when I got to the last part of the level, got my path a little messed up and was able to restart that part of the level instead of having to start it all over again, it'd be ok. Hell, if the level checkpointed at each screen transition, it'd be ok.
Regardless of the number of possible fixes to the last level, I would have only taken off half a star if not for the fact that because the last level is basically impossible, I can't access a decent chunk of the content. So yeah, 3.5 for you. And seriously, pretty kick ass game besides the end