Awesome game, highly recommended. The puzzles are well-thought and challenging, but rewarding at the same time. And there's a shitload of secrets to discover, ranging from backstory info to items and challenges. And, IIRC, they have just released the level editor for free, so the replayability of the game has increased many times.
When it comes to GOG VS Steam, they're about equal in my mind. Sure, I love having a gigantic, easily accessible catalog on Steam, but GOG is DRM-free with a no-nonsense install process (except the sometimes infuriating bundled software in their game installers, but that's Foxit Reader, which is actually good).