Overall, neat concept and good execution - the art is nice as well. I wasn't exactly sure what to do at first, but figured it out after a couple minutes, and the UI was very nice.
I ran into performance issues though, buy not immediately. At first the game ran fine, but over time it slowly degraded. it never crashed, and I was able to progress through several levels, increase my play area, etc. but the audio noticeably slowed down as well as the UI. It did this just looking at the intro screen as well, so I don't think it was just because I increased the world size.