"Ten years ago I bought a bar of your soap. Since then I have used no other". There was a famous cartoon, probably in Punch (in the early years of what is still this Twentieth Century) - the illustration that went with it showed a fiercesomely unwashed tramp writing this unsolicited testimonial letter to a soap company in the hope of getting a handout.
That not wholly relevant, but Mark Clark's remark about Webferret "I haven't used another service since", made me think of it. It may have been me that put him onto it - I make a point of doing so at every opportunity, because I think it's brilliant. It always seems to come up with the goods within seconds. And it's free, which is always useful, though I believe the version you pay for has some useful extra features.