It beeps when a car is X distance behind the bicycle, so you know it's coming up. Apparently, if you have a Garmin computer, like Rachael, you can also see it on the display.  I wonder if it shows what sort of vehicle (car, truck, semi). Jacinto just gets the beep. 

I don't think it would be as useful for me, as I have a mirror. Perhaps if electric cars get to be more common? 

The flashing light feature is optional, in a different model. I was disappointed in the brightness of the light. Jacinto says he has it as bright as it will go. 

I'm trying to think of the last time I failed to hear a vehicle overtake me. It was probably 20 years ago on an extremely windy day.

My local area has calm winds and light traffic, so I can usually hear an approaching "stock" vehicle 30 seconds before it passes by. About half of the vehicles in my area have modified exhausts that allow me to hear the approaching vehicle a full minute in advance. My tour routes intentionally avoid super-windy places such as the US Great Plains in spring.

Quieter electric vehicles will be a problem eventually, but aren't even a minor problem for me now.

I use a mirror and rear flasher all the time when cycling, and notice that vehicles pass more closely when I forget to turn on the flasher. I just ordered two Cygolite 350 flashers for free, paid for with Capital One reward points. It will be my first time to use two rear flashers.

Lights have come a long way over the years. I think you will be happy with the lumens for the dollar with the Cygolite 350.

I also run two lights. I have one on my rear rack and one on the seat support of my recumbent. I set them on different flash patterns, to be more noticeable. I try and space them as far apart as possible, which isn't that far. But I tried. 

My original hope was, on long touring days, that at least one light would still have life when I got to town.The last day I can readily remember both of my lights running out was between Dillion and Jackson, MT. It shouldn't have been that difficult of a day, but the wind was fierce. 

I've had one for 3 years now and I love it. I still use my bar end mirror and still look before I turn but my situational awareness has been vastly improved.

The integration with my Wahoo Element bike computer makes it very easy to always be aware of what is coming behind me, same integration with Garmin and most other units.

The biggest benefit for me on day rides on country roads has been the ability to stay a bit more to the left (on US Roads) - gives me better visibility and more reaction time for varmints scurrying out of the woods/bushes from my right - which is my biggest danger on most rides. Similarly, on more urban or highway-ish roads, can do the same and avoid a crappy shoulder with more warning time to move right.

It comes with seat post and seat stay mounts but those might not work for many/most touring cyclists. There are many sources of innovative Garmin mounting adapters.

A few negatives:

• My touring these days is using bikepacking bags and it is not easy to roll up the seat bag just right to get the unit attached in a level manner. I've tried some velcro approach but don't trust it yet. On a rear rack or a trunk bag, an easy thing. 
• On service roads (roads next to but separate from a highway or other main road) the unit will alert on traffic on the main road if the two roads are right next to each other.
• If a car stays behind you too long at your speed, the unit will stop beeping about it. The display on the bike computer will still show it, but I'd rather the beeping continued.
• For urban riding, there are obviously always cars behind you and it is pretty useless - easy enough to ignore or temporary disable.

Thanks. Great information!

Thanks for the info, Kelly.

Thanks from both of us for all the input.  We’re convinced and going to pick up a couple.   We’ll report on our experience in a post in our current journal once we’ve reached our conclusions.

Scott - if you go to Shapeways and search on Varia,  you'll see a variety of cool mounting options being sold by people who designed them for fab on their own 3D printers and now sell through Shapeways.

Looks like prices have gone up (surprise!) since I bought my adapter, but here you can see the saddle bag loop/clip mount I bought and use.

There are all kinds of other options on there as well.

I'm sure this thing is wonderful but I don't understand why anyone would ride without a mirror.  It takes a split second for me to see everything behind me and I can check as often as I want to.   A flashing red light on the rear is also a good idea.  

It’s not a replacement for the mirror, it’s a supplement.  I rode without a mirror for many years.  I grew up riding without one, and didn’t see why I needed one since it just took me a split second to look over my shoulder.  I was wrong, as quickly became obvious to me as soon as I finally broke down and got one.  I’m pretty sure that will be the case this time too.