Actually, I have a picture of my bike which I accidently took with a flash which made the tire light up, then I decided to take a close up picture of it!

They definately light up and are visible at night time and will be noticed. Maybe there are different brands with different reflective 'qualities' out there ... mine clearly show up, not just in the picture, also out on the streets. Like Chephy was saying, if you bike along a sidestreet or even worse, an alley, it might be to late; I certainly agree, but this scenario could happen in daylight as well. I find that they light up from wide angles and therefore makes you visible approaching and coming through intersections from a driver's perception.

If you commute in the dark a lot, I'd say give them a try, neither you nor traffic around you will be dissapointed.