You can absolutely check your chain with a simple metal ruler. For me, this tool is worth it, as I have four bicyclists in my household, and a couple of more bikes than that which I regularly maintain. Also, I worked on a lot of extra bikes this summer, helping refurbish them for people, mostly kids in need, and having a gauge that quickly estimated how worn the chain is, and whether the bike could go out that way or needed a new one was a very quick and easy task with this tool.
This chain checker seems to give the most accurate sense of when to replace a chain. I have two bar-style checkers, which measure different from one another. This one reads closer to the Park CC 3.2 and always reads out above the recommended limit for Shimano chains just about when the chain gets noisy and can't be quieted by lubing. It's sold at descent price and may save you on bigger bills for replacement cogs. Keep riding!
The options are simple. Either you go to the bike shop when you think your chain is at the end of its life to get it checked or you spend a few bucks and buy this tool. Going to the bike shop is always fun but it will cost you time and money (not including the money spent while at the shop just because you are there and there are lots of cool stuff to buy there). Besides, having the tool handy means you don't wait till it is too late to change the chain and you don't risk ruining the cassette and even the crank.
In general, Park tools always seem well built. This chain checker is more than just a 'pass/fail' tool, it gives you an idea of how the chain wear is progressing if you use it several times over the life of the chain. The point of this tool is to replace your chain before it causes undue wear to your cassette - which is of course MUCH more expensive!
I generally take good care of my bike chain with a cleaning and lube every 2wks. during riding season.
I had 4500mi. on my last chain before I bought this checker and found I still could have added a few more
miles but I replaced anyway. This tool is very easy to use. Happy with my purchase.
It really is made to check the stretchwear of your chain, so it's appropriately named. Very simple to use, and it appears to give a precise measurement to let you make a decision about replacement of your chain. I haven't used the other chain checker from Park (CC-3.2), but this one seems much more accurate by design, so I opted to spend a bit more for it. Park Tool tools don't typically disappoint, and this little guy continues that tradition for me.