Quite good!
I enjoyed playing this a lot. It got a little repetitive after a while, though. Think of ways to bring in new content on later levels, it can be simple tricks - maybe by just changing the artwork a little bit or add new blocks (an empty block with no pipes at all will be a good challenge). The difficulty level is good, the artwork was good, very good job overall, just think of what to add to keep the player interested longer than 15 minutes.