It's really easy to use and comes for free if you own Portal 2. To play someones map you apparently have to subscribe to it from a link provided and it will be ready to play the next time you launch the game. Once you finish it you can rate it.
I've been playing around with the editor quite a bit recently but my first map was too big so it ended up really dark and unplayable, despite overusing lights everywhere. Stupid "unportable" black surfaces. My other map crashed before I could save so, I don't have anything to show at the moment.
Anyone else get a 2-3min "rebuild" time? It's really in the steep end for me.