I've got one, one of the new RTL515 models with both the light and radar. I've had it the grand total of 48hrs and one ride only so not exactly a lot to report, but here goes:-
1. Garmin assume you're going to mount the light/radar on your seat post and provide several types of inserts for different types of post, however seat post space can get crowded and if you're like me and use a rack & rack bag or maybe you use a proper saddlebag or even a wedge bag, space back there can get pretty crowded. My solution was to buy a small bracket / uplift kit that raises the light to just underneath the top of the seat. This give the radar bit a clear view and the light bit a nice and visible spot.
2. Switching on / off etc, dead easy and not too many choices.
3. Pairing to my Edge 530, dead easy and not too many choices.
4. In use, as I said this is based on one ride only (which was with a group of five other riders), yep it works just like it says on the tin. You get an audible bleep when the radar detects a car and a yellow warning band down the side of the GPS unit with a white dot to represent the car (or cars & dots as appropriate), those dots move up the screen as they approach you and that band changes to red for the last bit. Look on the internet if you want a full review, but it does work, it does give you a good sitrep, but it doesn't replace a look over the shoulder.
5. As a rear light, bloody good, or so the guys behind told me.
6. Downside, I did, I think, get a couple of false warnings, but that was only when I was taking my turn at the front and both I and the radar was looking back through five people. Maybe that's a few too many bodies and tough to scan through or maybe the car turned off before I looked, dunno, time will tell.
Overall I'm reasonably impressed, as I said, it doesn't replace a look of the shoulder before you make a manoeuvre and in town the number of warnings would render the unit a complete waste of time, but in my rural environment it seemed to work as advertised.
