i have a front knog light, and i really like it, so i got one in the back too. I decided to go for the bigger version though because i do not trust the drivers. Britain seems to be the worst place when it comes to giving cyclists enough space on the road. So I got the skink, more lights = better visibility. so far it works great. these lights are really bright and i love how easily they fit into place.
the only down side is that the skink is larger than the other knog models of lights, which means it doesn't just fit in your pocket when you park in town and want to take your lights off. (theres is nothing more stupid than when someone steals your bike light) but i bought the larger one on purpose, and I cannot fault it. plus, it looks much cooler than any other bike lights i've seen.
I cannot say much about durability, I've only had it for a month, but it does seem to be water proof, at least against heavy rain that is.