I received the chain wear tool from my mom as a stocking stuffer and it has been a fantastic gift. It is a must if you are going to be putting on the miles, if I ride a week with a worn out chain I’ll end up replacing the cassette as well as the chain.
The best one and most accurate I have found is made by Parktool.
http://www.parktool.com/products/detail.asp?cat=5&item=CC-3
