I have a Nokia Icon, and that is indeed my only complaint. The rest of the phone is great.
It's hit or miss. There's a ton of apps out there, even apps you wouldn't think would be out there. I was surprised that Plex and Teamviewer had Windows Phone apps.
Where it falls apart is apps you would think would have a Windows Phone app, but don't. For example, Snapchat or Yik-Yak. Or any Google-made Google apps. In the former cases, the story is even worse than it has to be: for example, there were third-party Snapchat apps, but Snapchat demanded they be removed from the store and still has never made a Windows Phone app. That kind of behavior almost makes me think there's a conspiracy among major app developers to kill Windows Phone. :)
Another area of weakness, of course, is the really small/local apps. My credit union has an Android and an iPhone app, but no Windows Phone app. Same deal with my vet.
I haven't upgraded to Windows 10, because I understand some of the great things about WP 8.1 are now gone. But I think it's really sad WP hasn't done better than it has: it's a fantastic platform devoid of apps. MS really fell down on the job doing whatever they had to do to get major developers on board, which, I think, has mostly been a social issue. From what I've looked at it seems like a great platform to develop for, though I've never made anything but toy apps.
As a former iPhone and Windows Lumia owner, the Passport is a huge upgrade. Great to read on. It's like using a Kindle or Kobo.
So: great interface, but no traction as a platform.
(I've never used it myself. I went from a Blackberry to a Samsung S3mini and now S4mini, and I'm still in "this is so much better than Blackberry" mode.)