Christian,
They simply won me over. I listened to others who owned them and learned (a bit) about tinkering and tuning them. To this day, when I share my experience with them, I try to warn that they aren't for everybody. IME, out of the box, they're crude and too loose/too tight lock rings are one of those things I find that I need to fix to make them really good. I gift a lot of them, but not until I work out the kinks.
The knives themselves also won me over. I listened to my hands. They didn't hurt when I pushed the knives hard. They couldn't feel blade play when I abused them (as my Bucks have developed). And my hands went through wood and pretty much everything else better.
<petty nationalistic wrong-headedness>
It is with a bit of shame that I admit to not wanting to like the Opinel. I worked for many years as a bike mechanic during the 70s and 80s. One of my great frustrations in that era was the French bikes. In that time, the French went their own way in terms of standards and the end result was working on bikes with no good options in terms of replacement parts. I came to regard "French engineering" as something of an oxymoron.
Now, I admit this was stupid, petty emotionalism on my part and I'm not at all proud to admit this. It's also why I tried so hard to like the Buck's that I've owned over the years.
</petty nationalistic wrong-headedness>
Despite all this, the Opinel has won me over. It joins the ranks of the "stuff that works" list that I keep. Svea 123 and Trangia camp stoves. Rottefella Super Telemark 3 pin bindings. The SunTour Vx rear derailler (or now, the Shimano Deore). The Weinmann/Dia-Compe 610 centerpull brake. Simple, mass produced designs that have earned their keep.
Consider me a zealous convert.