Definitely a fun time!
It's quite a solid tower defense, mechanically. And I'm 100% in favor of supporting indie game developers. I bought it, the play was good enough and judging by the size of the map I'll easily get my time's worth.
That said, I have to wonder if all aspects of the game are up to the challenge of attracting buyers. AT $5 with the Newgrounds coupon, I had no problem steping up. When the price goes up to $10 later, I'm not sure I'd feel the same. The art in the full version is crisper than in the demo to be sure, but I've played a good number of free games that brought better to the table, many of which on this very site. There is also a huge selection of similarly priced games that far outshine this one in appearance as well. The same applies to the lack of voice acting.
Speaking of voice and audio, I do enjoy all the music I've heard so far, you made some good selections. However there are a few sound effects (From the healers, notably) that are rather hard on the ears, in my opinion. I know there's an option to soften/mute sound effects, but I do enjoy having the other sounds.
I can't say I know how much time, money, and effort would be required to improve these aspects, as I've never developed a game myself. But on the player's end I know there are many products which do deliver it.
Don't take this to mean it's not worth buying, though! Play the demo! If you enjoy it, I encourage you to support the developers and drop a couple bucks on the full version. There's quite a lot more here to enjoy if you got a taste for it from the demo.