I bought the radar tail light for my wife who does not like to use a mirror. The problem is that she has a short inseam and there is no room to mount anything on the seat post of her bike. Why Garmin thinks everyone has plenty of room to mount things on their seat post is a mystery. I bought this so I could disassemble it and use the the twist mount to create a clip mount that fits on the loop on the back of her seat bag, like most bicycle tail lights. By the way, you can buy a decent tail light, with clip mount, for only a few dollars more than this product.
This product is a total game changer. I really don't know how I ever rode without one. I live in Houston and it can be a bit scary at times getting buzzed on the surrounding country roads. The Varia lets you know when a car is approaching so if you're taking up a full lane, rotating in a paceline, chit-chatting side by side, etc. it allows you to time to make (or not make) a move. The integration with my 520 is perfect and not visually distracting or cumbersome on the screen. You need one!!!!