Not a bad little gadget.
I own one of these.  It seems to be some kind of smooth rubber, so it's flexible.  I've tried the plastic ones and they're terrible.  You have to give it a few tries before you make your decision on the quality of the product because it takes a few fails before you get the hang of it.  It does work and it's pretty secure.  I had a bit of a difficult time when I first got it because my hair wasn't quite long enough.  Let's just say that arm pit length is about as short as you want your hair to be for using this.  Shorter than that and it will be difficult to use (althoug it may still work).  However, the little clasp is a plug style and it can be difficult to connect sometimes once your hair is already wrapped up, so I only give this four stars.  Once it's in, though, it stays.  Also, I paid about three dollars and the shipping was free, so you might want to shop around.
Rating: 4 stars