A bit dissapointed...
After reading all the raving reviews, I went ahead and bought this collar in a large size since my dog was right on the border between med and large. The collar is definitly very big.
My main complaint is that the collar is not quite as bright as I thought it would be. There is a single LED that shoots into a clear, rubber strip, thus illuminating the red strip (like a fiber optic). The major problem was that with the large size, the LED is not quite strong enough to light up the whole rubber strip. I ended up cutting off half of the rubber, so only half of the collar is actually lit up.
Other than that, a pretty nice safety feature for any dog that plays in the dark. I'd say, it does its job, but I can dream up a more spectacular product using multiple LEDs.