I just took my neighbor on a mountain bike ride today, and his bike... well, it didn't do so well. It's a full suspension 'Mongoose' with 20 inch wheels and plastic cranks.

His handlebars weren't tightened when it was put together, so in the middle of the ride they started to wiggle around. His rear brake never worked, and his front needs to have the lever pulled in to the grip to engage. He bent a crank arm and probably his derrailuer on some rocks, and he fell over and kicked his wheel, bending a spoke.
I do feel kind of bad for him, his parents refuse to buy him a bike costing much more than $100, so he has to suffer for it and save up to pay the repair bills. Every time I say anything about good bikes being worth it, he tells me I'm a snob, or some crap like that. I told him he should give up biking and do skateboarding, it's cheaper.