They are awful. And bordering on unsafe. The way a conventional valve stem works to keep air in our tires is twofold. First, there is the Shrader valve:
http://en.wikipedia.org/wiki/Schrader_valve
And on top of that is a plastic or metal valve cap, that serves two purposes. First, it helps prevent air leakage if the Shrader valve starts to fail in some way. Also, it acts as a protective cover to keep dirt and other contaminants out of the Shrader valve itself and causing it to malfunction.
The pressure-reading valve caps depress the Schrader valve so the pressure can be continuously read, completely disabling them. The only thing keeping the air in the tires is that screw cap itself, basically taking a two-pronged system and cutting its safety and reliability in half.
In our garage we have a $50 craftsman electric air compressor with a gauge on it (that is within a PSI of our standalone digital gauge), and I check my tires before leaving the garage just using the compressor. Basically every single time I hook it up, I need to add 1-2 psi, so just checking it each time without a way to add air would mean some pretty frequent trips to the gas station if I didn't have a way to add air locally.