My bike shop doesn't recommend the Park chain checker and suggested a Wipperman chain checker. I think it's accurate. The only thing that would be better would be to buy calipers and measure the chain when it is new and replace it when it has stretched by 1%.

Here is the Wipperman chain checker. There may be a plastic version too. Avoid that one.

http://www.bigringadventure.com/wipp...chain_wear.htm