I've been using a Lumia 735 for the last year (convert from iOS), and while I love WP8.1 (update 2, Denim), I've been eagerly awaiting the new 950 for its faster processor and superior camera. So the new phone came over the holidays. And I hated it so much after 2 days that I immediately returned it and have reverted to my 735. This was not a case of not liking the new 950 and its W10Mobile just because it's new or different, but because it's clearly (IMHO) inferior. Here's my summary of what's wrong with Windows 10 Mobile as run on the Lumia 950:
Phone -
Cheap side buttons, very poor feel.
Indifferent styling and colors.
Washed out screen colors & display.
Still has audio jack on top.
OS -
Despite nominally fast processor and ample RAM, apps often open slowly and there is lots of lag throughout user experience. Often worse than on my Lumia 735. (NB: I did a fresh install, not restored from backup.)
Poor use of screen real estate: Icons are crude and clunky given the size and nominal quality of display available. Especially bad:
OneDrive - much worse than WP8.1
Mail app looks worse, missing functionality.
Feels like entire interface has been dumbed down, made more childish
Icon and font design: muted and dull compared to 8.1. Also, fonts are often too small or too big, and UI is not as well integrated. In many places words are cut off at bottom of screen despite plenty of room available.
Contextual presses don't work or are non existent as compared to WP8.1. Overall feel is clumsy
Unlock buttons don't highlight when touched.
Settings are harder to navigate methodically: wrong headed attempt to show related settings doesn't permit backing out to same location you started, hence cannot work down settings in comprehensive way
Quick access settings are flawed: rather than taking you to, say, Wi-Fi so that you can select/change network, now the quick access buttons act as on/off toggles. To get access to the actual function you have to take a longer route.
Several settings that used to allow live tile views, like data sense and storage sense, are no longer available.
Many settings are no longer available to pin to start, eg. Call Block.
Edge browser: Dreadful and half baked.
Favorites don't transfer from WP8.1;
Missing functionality: Swiping back and forth between pages doesn't work.
Many other issues, like swipe writing doesn't work in Edge despite it working elsewhere in OS
Camera:
Despite supposed superior ability, nothing special is apparent (maybe due to poor screen quality, see above).
Non intuitive control of camera operation, eg. Live capture taken when it's not intended.
Phone and message apps: Often long lags in receiving notifications
Weather app: Now shows next three hours on live tile rather than next three days, not as useful.
App Store is dreadful, way worse!
Many apps choose hamburger menu convention when they don't need to. Older style felt more natural
Office suite: While it can be nice to be able to edit docs from phone, most usual use case is simple viewing. Now to view you're in editing mode by default - dangerous! It was better under WP8.1.
Some apps aren't available in new OS.
Thumbnail photos are now round instead of square in people app, example of stupid change for change sake.
Maps are terrible! Whereas old Here Maps worked well new replacements don't do same things, and details are washed out (inexplicable given the nominally superior screen). The maps look bad.
Music app, Grove: Maybe ok, but interface is very different and it wasn't clear that my music collection was intact or fully available as before.
Photos app: loses favorites, so can't use them for live tile. Not as good since less control over what shows on live tile.
Windows Hello isn't practical, should've been finger ID.
General comment: many of the clear deteriorations in the new phone OS are probably an artifact of trying to implement a single OS across all devices and form factors, which in turn suggests this single OS approach is flawed, at least with respect to use on a phone. I doubt Continuum will be more than a gee-whiz item, and so it's a case of the tail waging the dog, sacrificing phone utility for an imagined but not actual nor useful integration.
Reposted from Lumia 950 "unhappy camper" thread