I voted, but one option is missing for me; basically what @smatkovi and @poetaster already pointed out:
It’s a fully hackable GNU/Linux system, just like my other devices.
Having tried a few things on de-googled Android phones, there’s a world of difference. I can achieve what I need, in 99% of all cases without having to install an app, and I can troubleshoot & fix problems.
Android (and AOSP just the same) might be Open Source, but the mentality behind it isn’t
Mostly what was said ^^^^.
In another timeline I’d probably be running Meego 4.23 on Nokia N9-3000 Plus MarkVI Mini Hot Pink, but who knows, corporate spooks would probably have ruined Meego in the mean time.
When I chose SFOS, superior UI was at the same level as user controllable OS and privacy. Since 2.x UI lost its original offering, yet what persists for me is distraction free PIM, along with privacy and ability to tinker with the device.
I clung to Blackberry till fall of 2017, when it became extremely difficult to keep using. Privacy, security & not supporting evil companies was a huge factor. Don’t want to fund Google or Apple. Android compatibility was the huge differentiator as well.
Although my sim is currently on a Volla I still prefer Sailfish and I use that at home. I agree with all the reasons you mention and I would add that the design and the interface pleases me most. The pulley menu, the keyboard, the logical and easy to use file system, the local apps made by many enthousiasts in the past are all nice.
But what a shame that Sailfish has to be flashed each couple of years on a new device, now on a Sony with an odd ratio. Not everyone can do that and it doesn’t make the system stable. Sailfish deserves better.
Never make an app for what has a fully functioning web page.
I just never had an Android phone before the N9, and i liked it well enough to just keep going.
Android at the time felt extremely clunky, so switching then was never an option.
Then the privacy aspects developed (or surfaced?), so it was just another reason to keep on trucking. Another aspect of that is the sheer amount of spam you get in your UI. So many useless notifications and dialogs that you never needed - SFOS has none of that.
Then there is the aspect of it being a rel Linux. I can’t claim to lean on that very heavily, but it’s undoubtedly handy that you can actually fix things and build things for yourself to so much of a higher degree than in the walled gardens.
I assume this question stems from the rather off the rails discussion over in Sony Xperia 10 V - First impressions
It is a crying shame that apps, and closed-source ones at that, is the norm. But it is what people need to function in daily life unfortunately. Each person will have different itches to scratch. I do just fine with the 1-2 semi-mandatory apps, and i’d spend serious time and money on doing away with them if presented an opportunity, but until then i’m quite grateful for AppSupport.
I like the Sony with odd ratio and SFOS on it. Very good on reading.
I use sailfish os because I liked meego (I never had it in my hands because it was too expensive for me, it was beautiful) it’s a pity that nokia hasn’t given continuity in the development
I can access a majority of the OS via terminal. I can build apps I want. I choose what to update and when, unlike Android, you disable something, it enables it again behind your back.
With SailfishOS, I can remove or edit the offending app/file and never hear about it again. Freedom in the truer sense of the word compared to the other leading brands.
I bought into Jolla1 from the start and have no regrets. After 10 years, I’ve not physically met anyone else who uses SailfishOS, which is nice. I know SFOS has problems, but I can live with them.
One irritation, not with the OS but the SDK, I’d like to see the SDK getting some REAL love, it’s soooo buggy, at least for me, even with a fresh install.
I mean; yes! Security is lovely, and I trust our benevolent Finnish overlords, but GDPR has got nothing to do with this sense of comfort and trust.
My first smartphone (had a few nokia feature phones before) was a Nokia N9, but with Windows Phone OS. 
If I remember correctly, I used it for not more then half a year or so, before I found Jolla Phone. Immediately I bought one, and here I am. No regrets.
Sailfish also made me dump Windows on desktop over night, and I’ve never looked back. Of course, I was already familiar with linux, so the transition was pretty painless.
It must have been a lumia then, if it had windows phone
Yes, of course, I didn’t remember the name, but it is actually the same hardware. Microsoft just shifted firmware (and name) on the first Nokia Windows Phone.
Edit:
Well, almost exactly. 
My journey is similar. My last phone before the smartphone time was a Nokia Feature Phone. After that jumped directly on Jolla 1. In the same time i left Windows behind on PC and can not imagine to be forced to use it again In both cases I really feel, as if I am the owner, not the user.
Nowadays I’d rather say ‘the used’.
I became aware of the Jollaphone when pycage (Martin Grimme), the developer of the music player Music Shelf, reported in the WeTab forum that he would soon be picking up his Jolla1 at a release party in Finland (if I remember correctly).
After various Android and iPhones, I was curious about this Linux smartphone with this new user interface and in 2014 I became the proud owner and user of a Jolla1. I’ve never regretted it. It was cool that the UI was so intentionally different from Android and IOS.
After a short and intensive journey of discovery, I quickly got used to the very different UI.
The Sailfish versions with the 1 in the first place were something special.
Then I got a JollaC and an Xperia X, which I currently still use as my main device.
There is also an XA2 in case I need a newer version of Android support.
But I try to get by without Android apps if possible, which isn’t particularly difficult with the Xperia X with 4.4 ;-)…
Yes no.1 & 3 for me.
Interesting. The first Lumia was just a cheapened version of the N9 16GB.
I wonder if the body was the same hard plastic (drilled, not molded iirc) they used for the N9, because that was amazing.
Yes, it was the same body.
There’s an important option missing off your poll - I like to own the things that I buy. Smartphones are complicated machines, and often go wrong in some way. If I don’t have admin access to it, and I can’t fix something because I’m actively prevented from doing so by ‘security’ then that’s immensely frustrating. Examples of this are backing up whatever data I like, transferring any data I like to another device, finding out why storage seems to be taken up when the OS doesn’t give any indication nor option to free it without a factory reset, removing/disabling things that the OS doesn’t want me to. I’ve run into all these and more on other OSes and been asked to help with such problems by other people.
I don’t think I should have to jailbreaking nor root a device I’ve paid for and supposedly own just because the developers of the OS and apps upon it are arrogant enough to think that I would never need to fix something that they’ve done badly. The very idea that they would take that away to supposedly make the OS ‘simpler’ is condescending and dishonest. Let your users do what they want, with the provision that they may leave a supported configuration and require a reflash if it gets in too much of a mess.
As far as I know SailfishOS is the only option that provides both this level of condoned ownership plus an environment (with AppSupport) that can provide pretty much all of the functions that I would need in a smartphone nowadays.