I have a pretty good experiencce with O2 also, but I'm aware that it can be completely unusable in rural areas. Luckily, I don't really leave the city boundaries much.
EDIT: I just remembered, I used to live in Karlsruhe, and the Real there also had a self-checkout. I haven't lived there since 2016, for what it's worth. Yeah, not sure why it's confined to larger stores. Perhaps not popular enough to warrant sacrificing cash register real estate in smaller stores for it. Germans don't like new things after all.
As for mobile networks, I think all of them are frequently unusable in rural areas. Between Aachen and Cologne (I most frequently travel there), which is not a quiet route, I think at least half the time there is 2G or nothing at all. I should measure that some time. My work phone (Telekom) doesn't have better reception than mine (Dutch network using O2). Without measuring, I would actually estimate that O2 is the better deal around here.
* I've been to supermarkets in Aachen, Bonn, Cologne, Darmstadt, Hamburg, Leipzig, and Stuttgart (and many smaller cities/towns/villages of course, but I guess there it would be less likely).
> Good to know you're human as well.
:-) You should ask a random Dutch(wo)man for their opinion of the trains' punctuality though. We totally envy Japan.
The statistics are alright though and it has improved quite a bit in the last decade (much like how O2 is seen as having a terribly mobile net by literally every German person, but after the fusion with E+ a few years ago it is statistically fine, and indeed, I often have full bandwidth when my colleagues on Telekom are complaining). Public perception is a fine art!