The auto mode on the Infini works, but has a fairly dark definition of 'dark'. If you stick a couple of layers of masking tape over the sensor it's a bit more useful.
The B&M lights (front and rear) with sensor modes do work well.
How useful this is depends on your bike and how you use it, I think. If you're regularly locking up in public spaces or on railway station platforms and manually switching lights to 'off', it doesn't gain you much. If your light's mounted somewhere you can't reach from the saddle it might be useful. If you regularly ride through dense wood or tunnels, it's probably quite handy.
For my sins I have a 'brake' light on one of my bikes. It uses a PIC microcontroller and spoke magnet to measure the speed of the rear wheel. When there's a significant drop in speed, it lights up. After a couple of seconds of no rotations it stops timing the wheel revs and goes into use-bugger-all-power mode to save battery. It works, but I'm unconvinced of how useful it is. It was a Sunday afternoon project to test a new PIC programmer.
