If you ask people who have bought them, they'll probably say they are worth it. If you ask people who haven't, they'll say they aren't. No surprise there.
I fall into the "not worth it" category. To me, they are little like a "Car Bra." If you want to protect your lens/car so that it is pristine for the next owner that a nice gesture, but I'm not that considerate.
On the other hand, keep in mind that once you've invested in a DSLR and start lugging around a camera bag with two or three spare lenses, you have already crossed the line into dorkdom. Once you are on this side of the line, a lens skin isn't going to make you any more or less of a dweeb. So, if you think they look cool, go ahead.