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

I have gone threw around 20 to 25 chains in thew last few years and my preference has been Dura Ace chains with a fixed pin. My opinion is they take the longest to wear out (1.5 to 2x longer), quitest and all most never break. Good for training and racing.