It depends on what you are hoping to achieve by releasing it early. If you are just releasing it for the sake of releasing it, I would hold off. However the phrase 'test early, test often' definitely applies. If you are hoping to gather player feedback and use it as direction for improving the game, an early release is not a bad thing. In fact, I would say that 18 levels into a puzzle game may actually be a bit late to release and get feedback from players.