SpaceChem does it too. It's also great if you have people on your friends list that play the games, since then you can see their exact scores and turn it into a direct competition.
Yep, I've spent days tweaking and optimising the starting few worlds of SpaceChem to get solutions which are most optimal. The only disappointing thing is that you can only save one solution to each challenge, so it means you need to pick a solution that is either the fastest or the one with the fewest elements. Luckily they have fixed it with Infinifactory.