Defense Grid was definitely an amazing value at $2.50. I don't even like the TD genre that much, but I've spent 20 hours on it so far getting gold medals for each level and I still am only halfway through the challenges. The difficulty scale is also just about perfect - it's relatively easy to beat a stage but progressively harder to do so getting a silver or gold medal. Then it gets really hard and takes on a RTS flair if you're trying to get a high score because you have to let your resources accrue interest and spend only the bare minimum at the last possible second. Lastly, a lot of the achievements are a bit silly but fun (win an advanced stage with only green towers, etc).
Objectively, I would have been perfectly satisfied with Defense Grid if I had paid $20-$30 for it.