Garmin Varia RTL515 Radar Rear Light

It does what it is supposed to do.
It should have a lanyard as standard.

Prompt delivery at great price

An excellent bit of kit and especially useful when keeping a distance from the kerb to dodge potholes on the Surrey Downs. It's often difficult to hear cars behind you when you're travelling at 15 - 25mph especially if there is traffic approaching you on the other side of the road drowning out the sound.
It shows cars approaching from behind from about 140m away and dots (shows a queue of maybe up to 6 or 7 cars) travel up the side of the Garmin screen as they get closer. Screen goes red showing faster approaching vehicles. Plenty of online reviews to show how it works.
Only 2 gripes: the light (even on "Peloton" mode) is IMHO just blindingly bright and would be too distracting for following motorists let alone cyclists. I don't want to hear the excuse from my hospital bed that "I crashed into the back of you because I was blinded by the light". I've toned it down a little with some semi transparent tape.
Secondly the rubber band is (again IMHO) woefully inadequate in keeping the unit from rotating on the seat post (or maybe being detached completely). Maybe this is because I have a thinner 25.4mm seat post on my Synapse. I've replaced with a much sturdier band now. Important if 1) you don't want to lose £160 worth of equipment after hitting a pothole or 2) pull out in front of something because the radar is pointing at the hedge.
Plenty of reviews online. I found Tweeks were slightly the cheapest and delivery was very quick. Not cheap but now I wouldn't be without.

reliably picks up vehicles. Light also flashes when vehicle is close to ensure driver has seen you.

Gives me a lot more confidence on the roads. Gives you the full picture of what’s behind you whilst allowing you to stay focused on what’s ahead. Very happy with the purchase

Gives a really handy extra set of eyes behind when you have a lot to focus on.