This a good light to be seen, but not to see in dark conditions. It has a 180-degree view, but the light isn't focused so you can't use it for night riding. That's fine as I bought this to be seen. The only criticism I have is the battery. As it's winter, I haven't used this on a long ride, but it seems like 5-6 hours is the max battery life on even lower setting. It also takes a long time to charge. So don't use it at night and don't expect it to last on a long ride and it's fine.
With the accuracy of modern GPS computers the speed sensor might be considered obsolete but I still like using them for one reason. I ride enough bike paths and roads with heavy tree coverage that block GPS signals so I get inaccurate readings without it, call me crazy but I like looking at my current speed and knowing it is correct, there is a slight time delay but that is easy to get used to. The Speed Sensor 2 definitely gives me accurate readings that are verified when I pass the digital street signs that display your speed, the auto-sizing feature is a really nice touch as well.
I like every thing about my new 840. It's got so many features I initially had to turn some off or I'd be overloaded with too much data. It's quite an improvement from my 520+ which wasn't holding enough of a charge for long rides. i like the longer battery life and now find I only need to charge it after 3 or 4 rides. The only negative is that the buttons don't seem to engage after clicking them so sometimes I need to hit them more than once.
Coming from separate Cycliq Fly6 (commuter) and Garmin's radar/light (road) units I find the 715 a decent but slightly flawed upgrade for both my commuter and road bikes.
Garmin's radar is something I wish I had sprung for long ago, naively believing that my dorky looking but very functional (Take a Look) helmet mounted mirror was good enough. Still keeping a mirror on my helmets because nothing beats visual confirmation of what's behind me, especially with some of the more 'gifted' individuals given a license. The radar combined with a Garmin head unit (1040 and now the sweeter 1050!) just makes me feel safer. Functionally, the 715 radar works almost seamlessly, save for occasionally having sensor communication issues (Bluetooth or ANT+?) with the head unit du jour. The connection usually pops right back after a few nagging beeps and has yet to do so during alerts.
The camera portion is the flawed part, alas. No image stabilization is the real head scratcher from Garmin, given their otherwise well designed Edge and radar only Varia models, especially after years of iterations to get them right. Heck, I still run 10 year old Virb cameras on the front of my commuter that have stabilization! I bought this for safety (mostly so my widow has something to try and get compensation from whomever takes me out), not to record my epic commute, so I didn't expect action camera level. If I want that I'll get a dedicated action camera, and it sure isn't going to be rear facing. The lack of stabilization is something even Cycliq offers on their Fly6 models, at least of late. I have several older Fly6 models I used for safety, but they were never up to Garmin level build or support. One other nitpick with the 715 is that the lens material appears to be plastic or very soft glass, and there are reports of it easily scratching. I was able to find glass lens protectors from brotech that fit it, however getting them shipped to the US is problematic.
The 715 has flaws that make the additional cost questionable but worth it to me, given I get one rear device with lights, radar, and safety camera.