There's lots of reports out there that the Convertables in .357/9mm do OK and lots that say that they shoot tighter groups with the proper fitting .357 bullets. Some get around the issue by using the slightly larger cast lead bullets in their 9mm reloads. So getting a good one is partly luck of the draw and partly looking at options that make it work.
I've never seen the lure of it though. I suppose for someone with a few semi autos that wants to by ONE single action revolver it makes some sort of sense. But once you get a little or long way past that one lonely revolver then there's simply no big reason for a Convertible that I can see. And I passed that point long before I bought my first single action by getting a couple or three S&W hand ejector models. So I was already fully dedicated to both 9mm in my semi autos and .38Spl for revolvers.
If you're hands and wrists are a little crunchy then you'll love .38Spl in general and reloading for it in particular. Revolvers don't care how strong or light you load up your ammo as long as it doesn't blow up the gun. Semi autos, on the other hand, need enough power to cycle the gun. And I find that with 9mm vs .38 this means that some of the semi auto guns tend to snap at the hand more sharply than the revolvers.
This ability to tailor your loads to suit your personal recoil happiness is yet another selling point for loading your own ammo.
For sure if you want to minimize the shock from the semi autos you'll want to run an all metal gun. See if you can try out someone's CZ Shadow as an example. Or look at some of the 1911's in 9mm. If you'll be shooting in competitions like IPSC or IDPA then STRONGLY consider the Shadow. It'll let you sit down in the Production or Stock class with the rest of us bottom feeders
Also if you work on getting a set of cowboy guns you'll find that running .38Spl from a lever gun with the ammo that is suitable for cowboy action feels just a hair stronger than shooting a rimfire rifle.
Madcow and gdawg, thanks for the info on the Coonan. I didn't realize that they came with an optional spring. Are they a little fussy on the OAL? Or do you guys load the Specials a little longer or does the magazine engage the rims so the Specials sit back where they should be other than the one being fed?
I handled and shot one some years ago. But it was just a magazine's worth. My main impression was mostly geared to the big kick from a semi auto and the front to back depth of the frame that made it an awkward gun to hold securely.