My 6P is an old-school round one, so I couldn't replace the bezel if I wanted to.
I have a "soft-press" McClicky switch in it. Tried the hard-press based on reviews, but found it inconvenient for mode-switching duties.
I don't much care for twisty action, so I haven't tried a Zero-Res - but I am interested in the robustness and current-handling.
I run 2xCR123.
Currently, I have a 3-mode NailBender neutral XM-L in there.
When I get the M61 neutral hi-CRI Nichia 219 drop-in for my C2, I'll put my M61W back into the 6P - where it came from, and where it belongs.
I don't think I'll go back to the hard-press boot, however.
I have 3 Surefires, and in a week I'll have 3 M61's (W, 219, and L HCRI.) I'm going to pair them up as my "Always dependable, indestructible, never fail me" lights.
(The Low-output HCRI is in a G2z.)
The neutral XM-L will give new life to a SolarForce L2P. I do like it a lot, and NB does great work, at competitive prices.
For some of my other P60 hosts, I have (or am getting) a NB XR-E optic, EDC+ neutral XM-L, and vinhnguyen54 Nichia 219 drop-ins. These can be considered nearly as good Gene's - not as robust, but more mode and emitter options, and quite a bit less expensive.
I also have several of the SolarForce XR-E and XP-G drop-ins. They are quite good for the price, and I haven't had any functional problems with them, but they either have too many or too few modes and I lost the tint lottery on all but one.
I was going to try the LumensFactory Hi-CRI D26 module, but selected the EDC+ instead because I've bought some lights from Jason and wanted to give his drop-in a chance. I should get it next week, the same time as my new Malkoff - so I can't say for sure how good it is as yet.