Very neat mechanic, although I feel like it could have easily been a ball, there's a missed opportunity with the way boomerang move. Maybe you could have levels without walls?
A few UI issues: your text appears above the level, it's frustrating to not see a part of it after you through the boomerang. Also, the transition after you lose text way too much, I just want to skip it to start playing again. Having the boomerang indicate that he's about to change it's state could make a huge difference in terms of readability and perceived fairness.
Good luck!