I use prefolds about 50% of the time. I use them with Nature Babies guesseted wrap so never have leaks. Sometimes poo gets on the cover though but never leaks through.
They are dead easy to use and you can't ruin them easily like other nappy systems. Also cheap, so you can buy loads of them and never have to worry about running out of nappies. I find them very absorbent as well so I use them whenever we go out.
The downside is, once my DD started walking I've had to make sure she is wearing a vest every day when I use prefolds, to keep the nappy up, otherwise the whole nappy slides down her hips. I have ordered some different wraps to see if we can fix this problem, also I think some people pin them but I can't be bothered.