I got a new headlight for my bike from DiNotte Lighting. Last night was the first time I'd taken it on my regular commute. The biggest difference is how it lights up the side of my field of vision. The old 600L model put a nice beam on the ground right in front of me which is great for riding. I did appreciate the side view afforded by the XML-3. Another plus that I probably won't really appreciate for a couple months is the extended battery life. I can run the new light about twice as long at the same brightness as my old light.
The most obvious improvement is the quick release mount. It's super easy to take on and off and I don't have to move mounts between each of my 4 bikes to allow them to use the XML-3. Furthermore, the 600L was limited to about 45° sideways movement left and right of center (for a total of about 90°). The XML-3 is pretty much free spinning although that is limited by the cord. It's super cool that I can whip it all the way around if desired.
One more improvement is the first flash mode. The 600L goes: .....off...... .on. ....off.... .on. whereas the XML-3 goes ...dim..... .bright. ...dim..... .bright. so the light is never completely off in flash mode.