I don't own one. I remember when they first came out, causing a bit of a stir in the blackpowder community. It was big news when manufacturers like Browning and Ithaca decided to make muzzleloaders. I liked the fact that Browning did not call it a "Hawken," but instead chose to honor their own namesake. I couldn't afford one of those rifles at the time, and when I reached the point at which I could, they were out of production. Since then, I've passed up a few... Either due to condition or price. I've been too late to connect on a couple of deals, too. These rifles seem to be coming up for sale a lot more now. My guess is that a lot of the original owners held on to them, but have now reached "a certain age" and the old rifles are now up for sale.
The three complaints I keep seeing about these rifles are 1) weight,2) the single set trigger, and 3) difficulty getting parts for the lock. Maybe L&R should consider adding a Browning JBMR replacement lock to their RPL lineup. The weight is less than most original mountain or plains rifles, and I guess you either like the trigger or you don't. In his post above,
@Hillbilly Delux said he replaced his with double triggers. Sounds like a good solution for one who likes the rifle but not the trigger.
Anyway, if you type "browning mountain rifle," in quotes, into the search box in the upper right of your screen, you'll get ten pages of links to posts discussing the JBMR on this forum. Probably more information than you would want to read in a day, but it might help you with your questions.
I'm not a hard-core collector. Every gun I get is with the intention of shooting it. The going price for a JBMR on the collector market is one thing, but the price you are willing to pay for a shooter is another. I will say that if you hire a custom builder to produce a rifle comparable to a JBMR in pristine condition, you're looking at a price tag somewhere north of $2,000.
Best of luck to you!
Notchy Bob