I've done it several times now...all with ring slings. I do it because I used to wear my daughter all wrong and almost gave up on babywearing because I couldn't get her in the darn otsbh right. I read the babywearing section of The Baby Book over and over again, but it never helped. So, when I finally got educated on sling wearing, I felt almost like it was my duty to help people!
Not people who have it on their necks or have it twisted, but I've approached several moms who have their babies in the sling either so low and loose that the baby is flopping out, or so messed up that they have to hold on to them (like virtually sitting on the rings!). In every case, they have been soooooo grateful, even though I think it was weird at first. I get nervous, but I don't want them to give up like I almost did.