BlaBlaCar: A Review & How-To

Yesterday I took a BlaBlaCar from Valencia to Madrid. It was by the far the cheapest option to travel between the two cities, and for me was a pleasant experience that I plan on repeating between other cities within reasonable driving distance.
What the heck is BlaBlaCar?
BlaBlaCar is ridesharing community--the biggest of its kind in the world. You can access the platform through their app or website, where drivers publish trips they plan on taking and riders can request to join for a cost. Typically only long distance drives are available.