Do you know this game? I played the demo yesterday and I am addicted to it. Game so addictive and reminded me WarCraft III Tower Defense mode. I want to buy the game but it's price $19.99 and now this price is too high for me. I hope game will have a price cut soon, $9.99 will be nice. Anyway you can try the demo from here. And can get the game from Steam.
http://gamedaily.newaol.com/pub/DefenseGridDemo.exe

Well, I've just finished one of the last missions, so my comment on this game would be:
Awesome, addictive and the best tower defence I've ever played - and I've played quite a few.
This game solves all the annoying things about tower defence games I've seen, adds it's own things like line of sight, and throws nice graphics as a bonus.
I was skeptic at a start, but now I can say it's worth 20$ it costs. Totally.