Over all, only really using 10166 in depth (used 10149 for 3 hours then found MMS was not working and rolled back), I think I would upgrade. Why ?
1. Better Email client (worlds better than 8/8.1)and better calendar
2. Customization of the start screen is more dynamic, you can do a lot more with it (background images or just on tiles, or other options) to make it yours
3. Better Notifications with more shortcuts, even a flashlight (finally !)
4. Better Alarm layout (something I use every day) with stopwatch and timer built in.
5. Voice writing on any app, when you bring up the keyboard, it shows a microphone, was doing Twitter and facebook posts via voice, saves so much time ! Very cool
6. Better Xbox live Client (I am a gamer, so it's something I was looking forward to, although very buggy at this point).
7. More options in Settings, although it's not finished, I think it's got a lot of hope
Now, All depends if all the bugs are fixed will make the deciding factor, Cortana does not work right (only partly), Xbox live is slow as crap, Application crashes are very common, battery life is good but, not as good as 8.1 and the phone gets very hot after even 3-4 min of use.... So, I will not update on day one but, maybe day 3....
Then there is the other side of it, IF I OFFCALLY can. This is a Verizon ICON, a discontinued phone that we made a BIG deal to get 8.1 and it took forever. The MAJOR question is, will Verizon actually spend the time on a phone that was discontinued about a year ago with a new update ?
Knowing Verizon's history, I don't think it will happen, so we will be stuck on The dev preview forever, If it does happen, 6-8 months after release.
Waiting for the moment to say "S.C.R.E.W YOU Verizon again".....sigh...
but, no question when new models come out (high end models, not entry level stuff), with Continuum, I will look, as long as the Application "gap" is closed a little with Universal apps...