I carry a 642-1 in a Side Guard Holsters Quick Clip IWB holster.
Reasons?
It's the only gun I have right now. I kept it for it's versatility, which is funny because I can't even remember the last time I carried any other way than IWB. I've carried 2" and 3" K-frames IWB without a problem, given a good belt/holster combo (1.5" Beltman/Milt Sparks SSII or VMII copy).
Snub revolvers, particularly Centennials, have a lot of good attributes for a carry gun. The no-dash 640 has been my "grail gun" for quite some time, but I like a carry gun that's not too difficult to replace if necessary. One of the reasons I like the 642. But I think a steel J-frame would work better for me since I carry IWB nearly all the time, and pocket carrying even my 642 is a little awkward for me. I would probably go with either a 640-1 or 640-1 Pro Series, assuming no internal lock. I'd still carry my preferred .38 Special loads, though. But the 642 serves its purpose quite well, so I'm not in a hurry to replace it.
I chose my holster because it's excellent quality at a very good price; it's easy on/easy off; it's an IWB holster with a reinforced mouth that stays open so it makes practicing easier; the clip positioned behind the cylinder lowers the profile and pulls the grip into the body, making it conceal better; and the cylinder rides about even with the belt, which is where I like it.
Here's a picture. This picture has my previous 642-1, but my current one is the same.