Bought 1 ea. red, green, blue (flashing) and dove with them today. Bright and easy to see. Makes it very easy to keep track of your party. Useful even for day dives.
Construction seems reasonably solid (although sooner or later somebody's going to thump a tank down on one; not sure the plastic housing will stand up to that level of abuse).
As an unexpected bonus, these float - so if you tie them to your first stage, they kind of float over your head. Makes them easier to see.
We also got one of the Sea Elite light sticks - definitely recommend these over that.