Downloaded this puppy, and completed the first level of the story mode.
Basically, it's Tower Defence. It plays well, the touch screen controls work well, it's a bright, vibrant game world with colourful characters and a good sense of humour. Waves of enemies move snappily enough.
That said, it doesn't do anything to change up the Tower Defence formula, so basically, if you're a fan of Tower Defence and want something with higher production values than free games typically have, it's a good download. There is plenty of content to justify the price tag - it's much cheaper than a retail game.
If you're not a fan of Tower Defence, or, if you already own a Tower Defence DS game (such as Lock's Quest), there's nothing in here that will excite you, I imagine.

