I really don't like them, but I can see that they have their place. We had some for DD but didn't use them much - when we did we made sure we help her hand as well, so we were teching her to hold hands, and the reins were a back up. DS is only 14 months, so I'll wait and see.
Unfortunately there are a lot of people where I live who really make reinds look awful. They seem to hold the reins up in the air, really tight, so the child has no slack at all, and it looks very controlling, and , sorry, but like they are training a dog. I've also seen people use them in the strangest of places - eg, in a fenced playground where there are only a couple of other children playing, or in ikea (on a very quiet morning), and in both those case the children seemed to be bimbling along at a reasonable pase, not racing about wildly. I think its sad in those situations not to give kids some freedom to wander.