I haven't heard of any yet that have had problems with the K40, but that is not the case with the TN31. One guy posted at another forum that on his brand new TN31 he just received that he couldn't turn the ring without turning the whole head. That's not good.
I have owned my stock TN31 for quite a while and it is the greatest compact short bodied thrower I have ever used.
I have never had a problem with my control ring or anything else.
I don't think there has been enough reviews or posts on the L3 K40 light just yet to know what it really performs like or the quality of the build.
I also own the Saab modded TN31 and it too has not given rise to any problems except for what he will do to outdo the modded TN31.
It really does rip into the night at 242 Kcd with a throw of nearly one kilometer (around 1000 yards).
The Saabluster modded Fandyfire STL-V6 tactical style light I bought as well is quite a performer at 117Kcd for $98 and that is a bargain in my opinion if you can't afford his modded TN31.
It is over twice the Kcd of the stock Fandyfire of 48Kcd.
Cheers