I installed a P-Rails in the bridge position of a single pickup guitar, and I am in love with it.
I have wired it so that I can use it as a full humbucker, a P90 or a parallel humbucker. I was so satisfied with these three sounds that I have never even tried the rail side!
The humbucker sounds very fat and loud (perfect for solos or big stoner rock sounds, not really tight with modern metal sounds); the P90 barks with rage and clarity and is very nice on cleans too. What really stands out, though, is the parallel setting: much fuller than a normal parallel-wired humbucker, still has a nice attack which the series mode lacks.
Edit: I finally tried the rail side too. It's a nice (gentler than P90) sound, a bit weak if you want aggressive distortions, but nice for mid/low gain stuff