I'm currently using the Knog mid Cobber and it's been really good. It's programmable and has at least 6 different settings you can cycle through, but the biggest advantage over most other rear lights is that it is more visible from the side and angles than most other rear lights because of its shape. That said, I've been thinking about getting Varia radar once my Hammerhead bike computer ships (currently using my iPhone as a computer, and Varia only works with Ant+). The debate will be whether to get the Varia stand-alone radar and keep the Knog, or just switch to the Varia rear light for simplicity sake.
I will add that I recently bought a Knog PWR 1000-lumen front headlight, and it's pretty awesome. I'm totally comfortable biking in the dark now.