I like using music to keep focused while out riding, but wouldn't wear any headphones that prevented me from hearing what is going on around me. These allow me to still hear anything from a car coming up behind me on the road to a cyclist quietly announcing their presence as they pass me on a cycle path. Wind will get in the way of the sound, but any headphones will have this problem. These do need charging between longer rides, as I've found battery life at a volume high enough to hear the sound clearly (about 3/4 max volume on an iPhone) is around 4.5-5 hours, and the one time they've died on me it was fairly abrupt, but I had just arrived home. I would definitely recommend these for listening to music (and turn by turn if you are navigating with a phone) while out riding.
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.
I was a little skeptical, even after looking at u-tube videos. But wow, I'm very impressed with the technology. I bought it because I ride solo more than 50% of my miles (5000 - 6000 per yr), but it works equally as well with a group.
I use it with a Garmin edge 830. Love how it interfaces and shows me the approaching vehicles on my 830 head unit. Shows multiple vehicles. I have found it very reliable. Rarely do I get a false reading from another bike (only if our differential speed is significant). Definitely makes me a safer rider. Only wish - louder or adjustable audio alarm.
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.