As many here are aware, since this subject comes up on a regular basis, I have a S&W 360J that has been rechambered to 9mm (9x23) by Mark Hartshorne at Pinnacle. I wanted a lightweight 9mm snubby and no factory guns met my needs so I had one done up. I've been shooting and carrying my gun for 2.5 years now and I'm as pleased with it today as when I first got it back from the 'smith. It weighs a hair over 13oz, is accurate, superfast to reload, has more power than a .38spl of comparable bullet weight but much less muzzle blast and recoil of a .357mag. Is it perfect? No, there are compromises as I mention below, but the package as a whole works very, very well and, for me, the pros far outweigh the cons.
Anyway, I want to address some of BCRider's points based on my experience.
A .38Spl revolver may not be made with the right cylinder metal to withstand the pressure from a 9mm round. A .357 would be made with steel rated for such pressures.
Per S&W their .38spl and .357mag cyls are made from the same stock, no differences in metallurgy, just chambered differently. These guns and cylinders are stronger than people think. The S&W 940 did not have a special cylinder and it handles 9mm just fine and the 940PC was chambered in 356TSW, which generates pressures similar to the 9x23, around 42k-45k psi, and it used the standard 940 cylinder, the only difference being a slightly deeper chamber.
I have shot standard 9mm, +P, +P+, .38 Super, and 9x23 Winchester through my gun with no ill effects. Generally I train with standard 115gr FMJ, my carry load is 115gr JHP +P+.
Then there's the issue of the cylinder needs to be chambered for 9mm. That would require reaming and sleeving. If you only reamed lightly to allow the very slightly fatter 9mm cases to fit then when the bullet leaves the case it would fly unsupported over some distance before entering the smaller bullet sized portion of the cylinder chamber up near the front before then passing through the gap and into the barrel. During that time where it's unsupported between leaving the casing and entering and sealing in the forward portion of the chamber it would be free to tumble and for the propellant gases to bypass the bullet. Both of these factors scream for the cylinder to be reamed and sleeved to produce a better shape of cyinder chamber.
Yes, the bullet travels through hat 1/2" or so of "freebore" before reaching the forcing cone and some gasses are lost resulting in slightly lower velocities than you get from a revolver properly chambered for 9mm but it is a compromise that comes with one of these conversions. I had the forcing cone of my gun chamfered for a smoother transition from chamber to barrel.
Then the barrel is a couple of thou too big to get a good seal for the bullet and for the bullet to swage into the rifling correctly. So that would require boring and sleeving the barrel to get the most out of such a project gun.
People make way too much of that .002". It's just not enough to make a difference, the bullets catch the rifling and spin true, no key holing, no odd flyers. My gun is as accurate in my hands shooting 9mm as it was shooting .38spl before the conversion. I don't use it for bullseye shooting because that's not what it was made for but I have had no trouble qualifying with it and my scores were within a couple of points of my duty gun, and our course includes shooting out to 25yds.
So all in all unless you're fanatical about the idea of a moonclipped 9mm revolver the idea just does not make any sort of financial sense to get there by modifying an existing .357 gun. But if your thinking is "damn the expense" then I sure want to see pictures of the project. I would LOVE to have a moonclipped 9mm revolver in a K frame size.
So, if you really want a 9mm revolver for self defense use or plinking and the existing factory options don't fill the bill having a gun rechambered is a viable and useful option. Mark charges $225-$275 for the basic conversion depending on the gun and that includes machining the cylinder for moonclips, not cheap but no outrageously expensive either. I had him do other work to my gun so it cost me a bit more than that but as far as I'm concerned it was money well spent.