At first glance, this is a decent TD.
Each map has dificulty scaling from brain-dead-easy to challenging-enough-to-keep-you-on-your-toes all the way to so-hard-you-will-have-to-come-back-once-you-have-leveled-up-more.
The story is more engaging than many RPGs and shows great humor.
The achievements are a decent mix of "collect as you go" and "work hard to get this".
The upgrading and customization of your "towers" adds a lot of strategic depth which is only hightened by the weapons and armor you buy/upgrade.
And then you finish the game and find out that there are achievements to play with just the story dependant heroes (Hero Mode). And the challenge increases by a factor of 7.
And then you finish the game doing Hero Mode and find out about New Game +
That's where the game really shines. Because not only is there a whole lot more story, greater challenge in all the previous maps, more achievements AND extra maps... there is also Hero Mode +
And then there are all the bonus maps...
The game is bursting with content, oozing challenge out of every pixel and spurting out humor.
No. Don't wait for a sale. This is not a game you MIGHT want, it is a game you can't NOT have.
PS: There is also a sequel coming. Oh, and a free HD version for everyone who has the game.
These are the kinds of devs you support. Do it.