A friend recommended this to me and I bought one. I love it. For a techie product, I love that it just works. Easy peasy. It is a substantial safety improvement to see cars approaching from behind. It is really the only game changer in many years. I bought an Edge 130 computer to go with it, replacing an older computer, and it is great, too. The best thing is that 1) I can work it, and 2) it requires no sensors. No wires, no batteries to keep replacing. I liked it so well I bought my daughter the same kit.
There are loads of these lights. I picked this one because of it's relative brightness. I immediately noticed motorists noticing me more often. The light has several settings but if you are riding during the day the high intensity flashing setting is really the best option. The rubber band is a bit of a struggle to take on an off the handle bar but once secured doesn't show any sign of wanting to depart the bike. Charging the unit is also easy relative to other tech since the light plugs directly into one of those cubes. You don't have that annoying search for which cord to use. How long does it stay charged? I have gone on 3-4 hour rides and still had light. I am trying to charge it every other ride just as a precaution. You can press the "on and off" top button to get an idea of how much juice is left. In all respects I am happier with this light than my much larger ,heavier and harder to secure Nite Rider.
This is a great headlight. the mounting to the underside of the out-front mount is fantastic as it removes connections of gear on your bars. The connectivity to my edge 530 is awesome. turns on automatically, adjusts the beam intensity for speed. Combine this with the Garmin rear view radar/tail light for the best possible set up. I thought this light was so good that i bought one for my wife as well. looks great. The windows which provide light to the sides is a great feature for visibility.
Have been using an RTL510 for 6-8 months, now won't ride without it. Useful in ways I hadn't imagined: (a) on fast descents when it's too dangerous to look back, it tells you if traffic is coming up behind you, (b) tells you if it's safe to move left to avoid bad pavement, (c) if you need to turn left, it helps you tell if a clot of traffic is approaching from behind, (d) while it doesn't display the lateral position of a vehicle passing you, you should expect a close pass if it shows that a pass is happening close to a blind corner or crest, or oncoming traffic. Also, I was unaware that its flash rate increases when traffic is approaching from behind until one day in my car I came up behind a RTL-equipped cyclist - quite a light show!
Limitations: (a) in high-traffic areas it will alert continuously (I avoid areas like that anyway), (b) if you are cycling along a secondary road paralleling an expressway, it may pick up traffic on the highway, (c) on curvy or rolling roads, terrain may block the radar beam long enough for a contact to be temporarily lost, reappearing as it gets closer. You should double-check with a head turn when it's important, because the RTL gives a low rate of false positives (not critical) and a lower rate of false negatives (critical!) in certain specific situations. The false positives seem to be triggered by motion-detecting lights or cameras (I routinely get false alerts when passing certain facilities). Occasionally it may temporarily show multiple dots when there is only a single vehicle, or "miss" a vehicle following closely behind another. False negatives happen when an upcoming vehicle slows to wait for a safe opportunity to pass me, so that it is going at my speed, no longer approaching me. If that situation persists long enough, that vehicle may vanish from my screen, making an abrupt left turn risky. Have gotten these false negs a number of times.
All in all, the RTL is a huge plus, greatly reducing the amount of shoulder-checking I have to do, so I wobble less and scan the road ahead more closely.