Sometimes your only choice. To and from larger small towns. They stop often and go to little towns off the main highways. They are very slow, but you will meet the nicest people. May have someone singing for tips or selling magic elixir, please be kind , they are just trying to earn a little money to help feed their families.
Second class is also cheap.
Probably no rest room, but it may have air conditioning, but don't count on it. For short trips, these are also a lot of fun. Fast, doesn't stop as often, air conditioned, nice rest room, may have movies, the driver doesn't play his cassettes so loud. Sometimes a first class ticket is only a few pesos more than second class. Plush, fast, comfortable enough to sleep enroute.