I wrote a blog post (here) last year whether I should or should not upgrade my development from WP8 to WP8.1, and my conclusion was "no go". Basically it's not worth it. I believe that I made the right decision back then, because otherwise I will fall into similar situation again, Win/WP8.1 -> Win10.
I have been keep track of MSFT's app development for a few years now, and honestly I'm tired and a bit sick about that. Because every time MSFT announce something new, something they are really excited about, developers need to drop something they had been familiar with and rewrite the codes. Even worse, the "excitement" is short, the "new thing" becomes worthless in just maybe 1 or 2 years (such as WP7, Silverlight). Maybe that's the reason WP's market didn't go any further in the past few years - At first, MSFT pissed off customers (WP users) by announcing WP7 cannot upgrade to WP8, and then MSFT pissed off developers by keep introducing new stuff without backward compatibility. Myself for example, I brought 3 lumia 800 phones (the price of 800 is almost the same as iPhone back then) at the beginning, when lumia 930 launch, I hesitated and eventually I went with lumia 720 and half year ago I switched to iPhone. I wouldn't call myself a loyal user of MSFT but I start using MSFT's phone since Windows Mobile 5. I don't know too much about marketing, but this is really sucks!
Being said that, the good thing is that MSFT seems to aware of the problems they are having. I attended the Build Tour in my city yesterday. They start to demonstrate MSFT technologies on competitors' products, such as MacBook and iPhone; and I'm sure you heard of MSFT start to attract Android and iOS developers by supporting them in the new VS2015 (probably because they are losing their own developers??LOL); MSFT is having an open-mind more than ever. This is good but this is far from enough and I believe they know it.
Win10, after a few years' struggling, this piece of ....... thing looks decent to me, from a developer's angle of course (I heard some noise about the UI changes BTW). But on the other hand, as usual, every upgrade is painful, especially true for WP developers. I would rather not going too much details here, you may have a taste by reading this post: http://blogs.msdn.com/b/lucian/archive/2015/06/09/win10-apps-in-net-porting-from-8-1-universal-to-win10.aspx
In short, it's not that bad by comparing transitions in the past, because most codes and XAML should be still useful while for 3rd part controls just give it some time, they will be supporting win10 sooner or later. At the very least, the underlaying frameworks are not being changed; they are not turning the framework up side down this time like what they did in Silverlight to WinRT. But I feel lucky and happy that I didn't go for WP8.1 last year because that is simply not the final stop. While for this time, I probably will go for win10:)
PS. yesterday's Build Tour, free entrance, free foods, basically free of everything and you could get a nice Build t-shirt at the end, my opinion to MSFT would be a lot more depressed if it's not because of that:))))))