First Impressions from me:
Disclaimer: I have not investigated if I have the latest FW yet, I have F-GGH-2C-1006151
I really like this compared to my gh615 (with latest gh625 FW) it's a great step up. Here are some finding's:
Formfactor: Like it much better then gh615. My wife also likes it better but she was still more found of the form factor of her Garmin-305 on first look so she gets to sleep on the couch tonight
Screen: Small compared to gh615, quite ok for me but it is borderline, the quad-view helps alot.
Backlight: I like the backlight on th gh615 better the gh505 gives a spooky light but it works.
GUI: The quad view is great, like it. There are some small GUI glitches that need's to be fixed, the font is to wide and numbers get clipped in quad view and GPS position view. Popup messages are to fast and the text to smal, I remember this being discussed so this is probably already fixed in newer FW. I almost never manage to read it after a alert beep. I think the popup time and text-size on gh625 firmware was ok.
I don't like the extensive need to press "PAGE" to cycle by all items, I suggest rearanging the GUI so all that have a "full screen stil-page" are on top of each other, and selected by using UP/DOWN e.g.
Activety -> GYM- > Navigation -> Connect to PC-> Configuration -> Location -> Current TRAINING->navigation (if enabled)
-> = PAGE button
| = UP/DOWN button
Activety -> Location -> Current TRAINING -> navigation (if enabled)
Connect to PC
GYM/Trainer: I like this will try this on my next 100miles run to keep track of myself compared to total time. But I think the default, builtin values must be changed to something more usable to give a good example like 10km on 5min is not a god start value, it actually a very bad value as it gives me an impression that I need to add pace instead (5min per km) I think you should have 10km 60min as default, thats is numbers everyone can relate to and makes it quite understandable what you need to add. Have only used DIST/TIME yet.
Mechanics: Buttons is much looser then my gh615 it seems that they sometime misses a press specially DOWN-button but maybe it's just a feeling will test some more.
Clock-mode: It this where you get the spoky pixels? I'll try to stay away from this as I don't need it, have another watch for normal usage. But I like it for another reason, on some of the longer ultras I switch over to a normal watch for the last part when battery is near dead, maybe I can keep it alive in this mode for the last part of the race.
Linux support: Well it was not working out of the box using SportTracks (from mono) with the globalsat plugin as it is with gh625/gh615 I'll investigate some more currently I spent about 30s trying...
EDIT: Got it to work in Linux It just showed up as another device, I just created a link to make it show up like something the plugin liked see my Linux thread for details.