I think Build 10549 is amazing on my Lumia 1520!
What makes it different is scaling at 175%, it is essentially a PC in my pocket, with all the applications from Edge, Outlook, Weather, Finance, Store, TV & Movies, and even Groove and other universal apps transformed to their desktop mode. This makes the phone so much more enjoyable to use - better use of screen real estate. Edge now has tabs on top, like the desktop version. Money shows a desktop equivalent of my stocks (no mobile econo mode), TV is just like the desktop app, Outlook is so much more usable with both portrait and landscape dual pane views...
Even the notification center and the pop up error boxes look better, like they fit on the phone from a UI perspective (before they looked like bolted on after thoughts). The smaller fonts, resizing of the boxes - looks really cool. Totally digging it!
Cortana looks better with the scaling since it shows more, but still hope they redesign the Cortana UI with some color and make it look more polished.
Maps now has a two pane view in landscape mode! this is so cool...
It feels faster, the Camera doesn't lag, and no more "loading..."!!
I can't wait to pick up the 950XL or the Surface phone, but with this build I might wait for a pen enabled phone (or at least update Onenote with stylus support!!).
I can't believe how cool the Office apps are to use now with the screen scaling! Need to now get my work phone (HTC One for Windows) changed to a 950XL and ensure our CIO enables Windows 10 mobile support as soon as it is launched.
This was my vision for Windows 10 Mobile - glad to see that MSFT shared the same ideas.