I picked up an India Pattern Brown Bess second hand from a re enactor. It's the one sold by Loyalist Arms. My impressions of the lock are as follows:
Cons: Fit is sort of sloppy and the parts aren't finished very well, especially the internal parts. File marks, rough finish, etc. The biggest con is the trigger pull. The trigger pull is so hard, it's ridiculous. You have to squeeze so hard with your right hand, even rudimentary aiming is difficult. The mainspring and the frizzen spring are way more powerful then necessary.
Pros: The thing sparks like crazy. It throws so many white sparks, you can hear them hiss. This is one of the quickest firing flintlocks I own. The tumbler, sear and bridle, for all their sloppiness have been hardened nicely. A file just slides across the surface. The frizzen bears no marks, gouges or streaks from firing. I've fired it very little, but so far flint life seems above average.
I would never consider using one on any musket I might build.