The key words with Contraption Maker at this juncture are "alpha", "early", and "access". Well, those last two go together, but you get the message. Contraption Maker is very, very young in its development cycle, resulting in a few unannounced crashes, inconsistent UI elements, and more than a fair share of miscellaneous errors. This is all to be expected, however, as not only is this the alpha release of the game, but it's also a complex game to develop. The slightest tweak in how a pair of scissors cuts something can break entire groups of puzzles.
The availability of official levels is in flux, as certain stages are pulled from time to time so the team can make more seamless adjustments to the game. The creation lab and community puzzles are out in full force, however, so there's still plenty to play around with. Creations you so carefully craft might not work in the same way from version update to version update, so try not to get too attached to your rotating cat cannon pinball machine.
The saddest news of all is that co-op doesn't exist in the current incarnation of Contraption Maker. The team is working on it, and it will at least involve both building and solving levels with friends, but the details aren't set just yet. Naturally, we'll keep close tabs on the game and throw up a signal flare or two once you can get your buddies in on some cooperative conveyor belt basketball tossing action.
Contraption Maker is still a little shaky on its newborn legs, but you can tell it's going to be a juggernaut of a game. Already it feels like a true successor to The Incredible Machine, only more refined and easier to play. And with Steam Workshop support, you can bet the flow of user-made content will never stop.