I've been using wool drier balls since this thread (about a month or so) and am happy with the results.
I don't like using anything scented but I do like prewashing my fabric, which I prefer to mix in with my regular laundry. I feel the yardage comes out in better shape and requires less ironing, as do the clothes. I don't have any statistics and maybe I'm happy because I'm watching the laundry like a hawk and it isn't the drier balls -- but there have been no problems because of the drier balls. I think there is a benefit and as someone who is bothered by many chemical scents, that's a good thing.