Fun, but has balance issues.
Honestly if you're just going to play this game through it's great. The ninja's can be annoying but you learn to compensate having a reasonable end of the line force while having something to hit them right off the bat to run out their invisible time. Great execution though for the most part, loved the towers and the upgrade system.
Only real problem is, if you want to get brilliant on every setting it's just not a player friendly enviroment. Some of the boss ninja's move so fast that even if you hit them right off the bat, it's impossible to kill them because they get to the gems before becoming visable. The only possible solution is to set up towers so they all hit the first unit coming out at the same instant, 1 hit KO'ing it. But then this isn't plausable for facing the normal waves up until that point, or you just don't have the money to implement such a tower set up before that boss arrives.
With a little tweaking though it could have been perfect and would have scored a 9 or 10 from me.