I really love the concept, and enjoyed it when I was playing. The levels are not very challenging though. Just a suggestion: Perhaps make the timed stars a little harder? I rarely ran into any difficulty beating par.
Pretty uninspired; I've seen this same game as a different name/theme a hundred times before. There was nothing here that really made it stand out as compared to others of the genre. That being said, it was still fun enough to finish all 50 waves, so I'll give it a 3/5