I bought a pair of these (each item contains one valve) and used these on a tubeless conversion on my 'cross bike's Mavic Ksyrium Elites. The front tire sealed up perfectly, but the rear would not, and would slowly leak about 20-30 psi in the course of a day. It got extremely annoying having to keep my tire pumped up all day, and I found out that it was due to the valve rubber grommet in the outer rim bed was ever so slightly too small for the valve hole in the rim, causing a slow leak that the stans inexplicably would not seal, no matter how much I put in the tire. I replaced the valve with a different brand with a larger grommet and now it works perfectly. The front (Mavic) valve is still going perfectly strong. It would appear the variances in the rim holes between both rims is the culprit.
However, these valves are nice because they lock into place using a washer. If your rims have relatively small valve holes, these should work fine, otherwise you may need a valve with a larger rubber grommet.
These valves include a full set of fittings for schrader, etc, but will not work for even moderately deep rims. The valves are only about 32mm long and will clear only shallow rims. However, once fitted they don't seem to cause any trouble. You need to make sure you align the valve to the rim bed.
PROS: Seem to work
CONS: Do Not Always Seal, Valve Not Long Enough, Difficult to Install
BEST USES: MTB, Cyclocross, Tubeless Conversion