Just what it says.... Got a heavy barrel Mod 10 Smith (owned by me since the early 80s) and thought
it would be kind of cool if I could use moonclips.
What do you think? If you had one done who did the machining
and were you happy with it?

38 or 38/357 revolvers are far more picking about moon-clip/ammunition combinations than rimless revolvers.  That said I have an N-frame 38/357and J-frame 38 both cut for moon clip, my Model 10 is not at the present. Both work pretty good once you have found good moon clip that work with your brass.  You really want to load one head stamp if you can.  Both of mine came cut for moon-clips from the factory.  That said if I was going to convert a revolver to moon-clips I would send it to TK Custom.

Why would you want to load rimmed cartridges in a moon clip? I don't see a real advantage over a speedloader.

I've read that it allows for a fast reload. Maybe, maybe not. I have no experience with moon clips....

Their isn't a huge advantage to the speed of putting the moonclip in, unless you're in a competition. Mostly because its semi-difficult to carry a moonclip in your EDC.

The advantage is when the moonclip comes out. Sometimes when you eject individual cases, they can get hung up, making a reload take longer. With a moonclip, they all come out at the same time with one easy push of the ejector.

ETA: It also eliminates the chance that a piece of brass gets under the ejector star. Which in a real life shooting situation, would be incredibly hard to clear under pressure.

The nice thing about having a rimmed cartridge revolver cut for moon clips is, if done correctly, you have the option to use it with OR without the moon clip.  Once cut for moon clips it will still function just fine without them.

Moon clips are faster than speed loaders especially in competition where they are carried on easy access belt carries.  They also are less prone to fumbles and malfunctions and thus also appear to me to be more reliable.  I have competed a lot with both setups and at least in competition moon clips rule.

The following link is to a match I shot last year.  I shot the stages with both my Model 10 fed with Safariland Comp III speed loaders vs my S&W 627 8-shot fed with moonclips.  There are a few other videos on my channel of revolver competition.

https://www.youtube.com/watch?v=2DIaPSL3eVk

For carry in my J-frame I carry with a moon clip in the gun for reliable extraction and have one speed strip for a reload because the speed strip conceals so much easier.

If you got the money it won't hurt and will give you options depending on what your are using your revolver for.

No....

TL;DR

Still can't justify the fucking up of a good Model 10 to modify for moon clips.

Can't you do this to a Taurus or Rossi instead? Something with zero resale value?

Model 10s have value? You can get them for $300 or less if you look around.  It does not hurt its functionality in anyway.  Moon clips RULE!  YMMV

Smith has made roughly 6 million of these guns since the original Hand ejector Model of 1899.
I'm not concerned with resale value as I have owned this gun since new and will never sell it.
Best 140 bucks I ever spent! I think moonclips would be good upgrade....

http://www.tkcustom.com/content/machine.asp

They will do you right when you decide you want it done.

THIS