Hi,
There's loads on this in 'Tool and Fabrication', but the low down is YES it's worth it. It only works on car stuff such as oil, filters, wiper blades, bulbs etc and tools (it doesn't work on bikes and cd players!). How much discount...... well it varies, bulbs are cheap, so are wiper blades and you get a couple of pounds off tools, but there's no hard and fast rule on the discount. Best bet is to take said product to the check out for a price check!
Steve