I used my Varia for the first time a couple of weeks ago on the Pacific Coast Highway, along the Sonoma Coast. It was great. The highway is narrow there and cars come up regularly but widely spaced, so they keep taking you by surprise. The handlebar receiver beeps and flashes when a car approaches, indicating speed and number of cars. It prompts you to look in the rearview mirror if you aren't already.
When we turned off PCH and into the hills on a lonely country road, we weren't overtaken by another car for the next 20 miles. It is easy to relax in that space and I was especially glad I had the Varia to alert me. In the city it would probably be beeping all the time, but I'd still use one now that I've gotten a taste of it. It really works. The rear light pulsates faster as cars approach, a useful city feature. It comes in two versions. One includes a little warning-light unit that sits on your handlebars. The other connects to your existing Garmin Varia-compatible GPS unit and costs less. It's the same Varia either way. The Varia comes with seatpost mounts. If you want to use it with a saddle or trunk clip, you can visit
Shapeways and order one.
I also love my Cygolite Hotshot Pro 150. That thing is bright! The mount differs between the 150 and the 200 models.
The 200 is seatpost/tube mount only. The 150 mounts onto tubes or clips into cloth straps.