I’m using Sailfish because it’s Finnish, secure, respects privacy, and yet allows me to do whatever I want! Having Android apps is a plus, but I could do without if I had to. And I do have a N900 in my closet still 
My phone doesn’t do jack if I don’t tell it to (I’m looking at you, Google Play), but I can bend it to my will if there are no other ways. It also doesn’t contain any unwanted applications (maybe the tutorial, but meh) which is a huge, huge win!
I think Sailfish is quite stable these days, as far as basic/critical use cases go (use it as a phone, sms, browser, calendar, contacts, web browser, and with including basic Android stuff, WhatsApp, Signal, Firefox, Vivaldi, Deezer…).
Some not-so-stable features would be Bluetooth support (there’s still fallout from bluez 4->5 transition, sound codecs, pairing issues), GPS (but when it works, it just works), VoLTE (beta, with high promises already) and unfortunately banking apps in the Android side… There are a few more, but can’t pick them up at the moment.
About user friendliness: I consider my sister as a total non-techie, and she decided to upgrade her Jolla 1 to Xperia 10 II, and not change to Android - it is suitable for everyday users! Soon there’ll be VoLTE in her phone, too, and the future looks stable 
Is Sailfish stable? In my opinion, pretty much yes. Far have we come from the prototype days!
Edit: Boy, was that offtopic
As for the question “why do I still use WhatsApp” is because there is one group who stubbornly won’t transition to Signal (and simply put, another group too), and there’s a conversation with an individual I do not want to lose. (Also, WhatsApp only offers exporting a chat to email only. Good luck with all the photos and videos…)
We aren’t really gaining anything, besides hopefully good recommendations about other people and groups we should follow. I can find my own groups of interests without an algorithm, thank you very much.