I've been using SailfishOS since 2014. Jolla 1, Xperia X, Xperia XA2 and currently Xperia 10III. I also have used Android phones and iPhones at work.

For some people the downsides are lack of apps. The few Android apps I use work just fine with the current hardware. Sadly I still have to use WhatsApp for a while, but for Signal there is a native app, WhisperFish.

The main thing to me is that SailfishOS is a Linux on your pocket. You can ssh into it, sync stuff with rsync or syncthing, edit your stuff with vim, have cron do stuff, or what ever you like. My old phones I use as remote sensors now.

There was a point that I tried to switch to iPhone. I struggled for a long time to get on par with the usability that I had with SFOS. I came pretty close, but the card house of different apps I had to build was pretty unreliable.

My phone is also my wifi hotspot. If I turn on vpn on my phone, then all the traffic from every connected device goes via vpn. I couldn't get iPhone to do this.

The team behind SailfishOS is pretty small, and regrettably shows in many areas. But still for me the clear winner of these three. It's not for everyone, but if you know your way around Linux it's great :)

So, not an Android or iPhone killer, but a good solid platform. The newest version 5.0.0.71 came out just a few days ago.