Edward W.wrote a review of Garmin Varia Vision on April 8, 2016
Familiarity: I've used it once or twice and have initial impressions
At this writing I have used this device for about an hour so I might change my mind. That said, first off, it was very easy to mount this to my Uvex 104 Sunglasses. It was also easy to adjust the screen so that it was easily visible without obstructing my view. The device readily paired with my Edge 520, programming the display was also straightforward (didn't need to RTFM).
While I do not intend to write a detailed review (as others have done so already, e.g. http://www.dcrainmaker.com/2016/01/garmin-varia-vision-hud.html), I will say that I found the device to be very helpful and easy to use while riding. So far what I have found most useful is to be able to easily track my cadence, speed, power and heart rate simultaneously.
This allows me to better pace myself (not work to little nor too much) to the extent that on the first hour out, on a day that I really did not feel like riding, I was able to improve my best 20 minute average power without *feeling* like I did.
Some will say that it is easy to look at the cycling computer when riding, but I find that there are plenty of cases when it is not so easy (like being in the drops in a decent or standing climbing at my limit and any especially at night). I find that being able to look at the display in such cases to be very helpful. Additionally, I find the display to completely readable in both bright day light (heading into the sun) or at night -- much easier to see than the Edge 520 display.
tl;dr -- I believe that for me, the Varia Vision will be a very helpful training aid and will improve my riding performance.
EDIT: The software update to 2.40 (that I was *NOT* allowed to decline) wiped out the Varia Vision setup. I don't think this is appropriate for a $400 device (plus the $300 for the Edge 520 that the Vision requires). If I spend $700 on a gadget that is not absolutely required to ride, I think that I should expect a bit better behavior. Some people don't spend $700 on their entire bikes...