My first mountain bike was a full rigid no-name cheapy. At the top of the first big hill I came as close to dying as I've ever been. I was out of the saddle going like mad and went until I fell over. I literally couldn't recover. I didn't know that it was OK to stop and push if you had to. The bike was massively heavy. It even had steel cranks! Those things weighed a ton. It shook me to death on the way down and convinced me to get a good bike. Who knew that a front suspension was a big deal?