Bit of an update after an ultra this past weekend that I used the C706 for - no real revelations or anything, more a confirmation of all impressions mentioned before:
GPS Quality
Flawless, even in denser woods or narrow valleys, no issues here - quick fixes after breaks too.
Data
Missing averages for longer periods (5m, 20m power/HR data) are reaaally annoying.
Maps & Routing
No news here, the "Unnamed Road" issue is annoying but doesn't really stop you from navigating correctly in my opinion. I tried the app's own route planning to check whether that fixes the issue, but it's absolute garbage and a nightmare to plan with, so I didn't bother.
Every now and then, the map rendering itself can be a bit cumbersome when encountering long road names for short roads, leading to the text spilling waaaaay past the limits of the road (akin to Garmin's web interface Leaflet integration).
Also, the compass tends to get confused on very low speeds sometimes, leading to it temporarily turning around and adjusting routing accordingly. Usually it fixes itself fairly soon, but it can also lead to a ClimbPro segment terminating.
ClimbPro
It's a bit of a mess, really. As mentioned before, it's far from a focus feature for me, but still - if it's there, I'd prefer it to work. It's fairly sensitive to some situations (termination on compass confusion or deviation, start/stop although that hasn't happened since) but more importantly, it's just plain wrong far too often. This starts with pure climb choice, ie. some smaller ramps that I'd have expected just not being classified as one while 20m inclines over 2km are then listed, and goes on all the way to profiles that are just so far away from reality: Far too often, you're riding a "green" bit of the climb while the grade is reaching double-digits.
Add to that the fact that more often than one it seems to also start/end the climbs in the wrong place and it just feels useless.
Battery
With a power meter and HR sensor connected and navigating a GPX file while mostly (98%+) using the map/climb view, 17h (and thus the lower limit of their stated battery life) was the absolute max.
Screen
As mentioned before, prone to glare but not too bad - as long as it's relatively clean. Get some gel or sugary drink on there and it's a different story. Not one that is easily read anymore.
Thankfully, most of these should be fixable with firmware updates, so hopefully they'll be on top of that.