I was almost finished building 5 when the game froze completely. Besides that, it gets too difficult too quickly. Puzzles like the lasers are one thing but the number of sentries so early in the game makes it way too difficult. If you prefer to game to relax, and dislike games that make you frustrated, avoid this one.

EDIT: Upon playing some more I have decided to change my review to positive. I stand by my above comments - freezes are unnacceptable, and a gentler intro would benefit the game. Making it clearer there are things to unlock and levels, etc, up front would also be nice. Once one gets more used to the game though it does become quite enjoyable.