I put 2 PRails in my Epiphone SG with the triple shot rings and it gives me a bunch of options for neck/middle/bridge positions and I've been very happy with it.
I just started working on my 3 P90 Strat again. I was thinking of making my own PRail type option and replaced the middle P90 with a humbucking Lipstick pickup and using the triple shots. So I'm pairing one lipstick coil with a P90 in neck and bridge, but also using both P90's/both lipsticks/combo of the 2 in the middle in series or parallel via 4-way switch.
Then there's my other S/S/S Strat with 3 toggle switches for series/parallel in the middle neck/bridge combo positions that I need to finish.