If anyone is upgrading from something like a 520 or 530 or 820 or 830, here are a few screenshots.
After getting the 840 and pairing to Garmin Connect on my iPhone, I ran a bunch of updates. Then added a most/all of the widgets to the widget loop. Here is my first pass at “new to me” stuff coming from the 520 and 530:

Looks like I have a 2.5 hour endurance ride and I can see current temperature and wind speed plus the forecast for next 3 hours. Tapping on today’s workout:

informative, but a pretty boring workout. Thats why I ride outside 
Tapping on the weather widget you can get the forecast:

Temperature, rain, and wind. Nice sunny afternoon here today.
Ok, back to the widget loop, lets scroll down one page:

again I added pretty much all the widgets, so we can see sunrise/sunset which I’ll either remove or move lower down the loop. You can see I’ve recently done 46 outside rides and 3 inside rides. Next is the Training calendar for the remainder of the week (yesterday it was empty - my day off). Tapping that widget leads me to this screen:

and tapping Training Plan:

those were all synced from TrainingPeaks to GC. Tapping on Monday and I can see whats coming up:

Kinda handy I guess, if I was on the road and wanted a reminder of whats coming up without having to pull out my phone.
Going back to this menu:

And tap on Workouts

looks like I can create a workout (the + symbol), but I didn’t tap that. Lets look at the suggested workout:

I pulled them up and they are basic workouts. Nothing wrong with that, just a little too basic for me after years of structured training. But a really good start for a newbie.
Back to the widget loop, the 3rd and 4th scrolls:

Those are all in the Garmin Connect app, but nice to see them here.

Cycling Ability is new to me, lets look:

I guess thats the nice way to call me a diesel with a low W/kg
and the analysis:

What can I do about it? Well lets start with Aerobic Capacity “higher intensity aerobic workouts” which I would normally think of as 5-min efforts, but that was in the Anaerobic Capacity bucket. And I’ve been focusing on increasing short power, even set a 2-min all-time power PR recently. So lets look at power curves for the last 3 months and scroll to to 10/20/30-min chart:

Yeah a big drop from 20-min (recent field test) to 30-min power because of the recent focus on short power.
Ok this is the ‘human powered analysis’ - maybe target 30 minutes? Lets look at recent 30 day load focus:

definitely been slacking the last 4 weeks on sweet spot and just doing maintenance short threshold work, while I did a rebuild on endurance and an anaerobic battery recharge.
Ok, for Aerobic Endurance it said I should put out more power (increase intensity) on long rides:

205 is the top of my zone2 / bottom of zone3, so I’m doing good at 2 hours and need to extend that out longer. Well the 840 is not showing any detail between 2 hour and 5 hours, so not going to do that analysis on the 840.
Back on a mobile or computer, lets look at the long end of the power duration curve. I’m pulling out WKO, but you could do the same in Intervals or TR or Strava GoldenCheetah or Garmin Connect or whatever.
definitely been slacking on the longer 2.5+ hour rides. And longer tempo/SS rides. Thats partly life interfering with training, partly a focus on short power to prepare for weekly Wed rides, and partly slacking on the longer rides.
Hope that helps anyone interested in making the jump from a 520/530 to the 840.