I am a longtime Garmin user for head units. My Garmin 810 recently started crapping out and I decided to do some research on the newest units out there. My expectation was to upgrade to the Garmin 820, but was surprised when I ran across this product.
What I love about this unit is the ease of setup and use compared to my Garmin 810. The companion app is outstanding and makes setup and configuration a breeze. The unit paired easily with my wi-fi, found all of my sensors (shows battery status of each, which is awesome), and was easy to install on the bike.
What really sets this apart to me from my Garmin 810 is how thoughtful all the menus are. For example, I ride early and turning the back light off and on is essentially a two button procedure. The post ride breakdown is not as detailed as other services (i.e., TrainingPeaks), but I actually like the way the information is displayed over Garmin's and TrainingPeak's apps.
I also love that you can easily adjust what you see on the screen while riding. If I want to focus less on power and just ride, it is easy to decrease the data fields on the screen while riding to one or two fields (e.g., speed, distance). Also, configuring data fields in the app is extremely simple and doesn't require all the menus that the Garmin 810 did.
I rarely use my head unit for navigation. I have not used navigation with the Bolt, but I have read other reviewers say that it is cannot do re-routing should you take a wrong turn. Others have also reviewed the navigation feature negatively due to the black and white screen, but I don't think this will be an issue for me.
Finally, the size of this thing is brilliant. It is small and light, and I prefer that over the heavier, clunkier Garmin 810 (even though it was a pretty good size too).
The one thing that may be a negative for some is the handlebar mount. It is plastic, and even though it installed easily, I can see some people stripping it in their tightening zeal. Also, I run a Arx II Team stem, and it is slightly (<2mm) off center. If you run a chunky stem, it would look weird way off center of the stem. Also, it can only be mounted left of the stem, so if you are a stickler about this.... The combo out-front mount and unit are supposed to be more aero, but if you choose to go another route, there are still plenty of great options (included stem mount, K-Edge, etc.).
I don't have the Garmin 820 or 520, so I can't compare this unit to those. However, what got me to buy this one over those was the reviews about the app (which I wanted after dealing with the Garmin 810 for so long) and the price. I also have to give a shout out to Kyle Brown at CC for helping me with the decision and purchase--Thanks!