Cyclo Chain Wear Indicator
This was the first time I had used a tool like this, always thought maybe a bit gimmicky, however, my opinion has really changed since trying it out. The instructions were very simple and helpful if you haven't used such a product - possibly could have figured out how to use without but I do like to follow instructions. Product really well package and was easy to use. Took it on a Breeze ride and tested all the ladies chains - something that none of the group had done either.
Small enough to carry in a trail pack, although not sure I would see it as an essential part of my kit, more in the workshop at home. £5.99 is the retail price and again without having tested it I probably wouldn't have purchased one, that said, I would definitely recommend to people to get one having now tried the product.
I don't think there is anything you could improve as it is simple in its design and use but gives the cyclist great benefits in showing wear and tear and a pretty essential part of the bike.