I use the light for daytime riding in flash mode. It is very bright and visible even in bright sunlight. The battery lasts for quite some time. I have done as many as 5 rides, 3 hour long, between charges. Even after those rides the indicators show that there was charge left. It is larger than my previous headlight, but it is better design over all. I have had very good luck with Portland Design Works products, and this one simply confirms my faith in the quality of the company.
I've been using Light & Motion bike light products for many years now. They are very well built, perform well and provide long service life. I use this light primarily as a daytime running light. The SafePulse mode is quite eye catching. However, I'm docking 1 star because it always turns on to the brightest steady mode instead of remembering the last used mode. This means it takes 4 clicks to get to the SafePulse mode.
I used to rely on handlebar mirrors and/or quick glances over my shoulder to check for cars behind me, but the Garmin Varia has been a savior and at least replaced the mirrors. The read out display on my Garmin head unit gives and audible warning and also provides ample alert to cars approaching from the rear. If there is any concern it is that while in group rides with teammates behind me the radar can sometimes confuse cars and bikes and filter out the warning, but perhaps I don't have the unit set correctly to accommodate for both bikes and cars behind me. I will not ride again without the Varia on my bike and should have purchased it when it first came out.
They should probably take "lightweight" out of the light description. The weight is my greatest complaint about the light. I like that it sits underneath the garmin computer. I primarily use it as a daytime flashing light and it's very effective for that purpose. It is plenty bright enough for later this year when I start riding an hour before sun up to avoid the Arizona heat. Attaching the light to the garmin out front holder took a little getting used to but overall it's no problem.