Park Chain Checker

There's a generally accepted theory that a chain will bed in and wear out much quicker than other drivetrain components. If you leave a worn chain on it'll rapidly wear out the chain rings, cassette and jockey wheels which isn't logical as they cost much more than a consumable and relatively cheap chain.

So how do you know when a chain is past it's best ? You can measure the length of 12 links (rivet to rivet) which should be less that 12 1/16", anymore and you'd best start looking to replace it. Another technique is to use a "chain checker", which is a little gizmo that tells you the wear without needing a tape measure. I use a Parktool Chain Checker (cc-2) which shows stretch as a %age.

A positive benefit of using a chain that's in good working order is that you'll have slicker shifting, with a drivetrain that doesn't skip, suffer from chain suck and last longer saving you money for more UGI (upgrade-itis).

Links; Sheldon Brown has some useful info on all things chains, including "stretch".