Everyone has preferences. I rarely carry a knife on my belt and primarily want a sheath so the knife blade is safed in my pack, or wherever it is stored.

I've recently picked up knife making for a hobby, and as a result ended up making kydex sheaths for them. I also have gone back and replaced the sheath on several other knives I owned (yes Doug, I replaced my Mk3 sheath; love the knife, hated the factory sheath).

For those of you wanting replacement/custom sheaths for some of your own knives, look into making your own out of kydex. There's more than enough resources online to easily learn how to make your own, and the materials themselves are cheap. I'd be happy to answer any questions if you have them.

I've used all sorts of sheaths. I think leather is the nicest, but I worry about it getting wet and about its long term durability. I got a kydex sheath for my Becker BK10, but found it clunky - and reinserting the knife was a two-handed operation. I worry about the safety of nylon sheaths - though I liked them for large folders - like the one Doug used to sell with the Mk1 (though I stopped using them about the time Doug stopped selling them - and I now just use the clip in a front pocket).

Though I'm sure Doug frets over them I have to say that the combo nylon/kydex insert sheath that Becker and Doug come with is honestly the best combination I've used. I like the choice of a snap on the keeper strap, and Doug's sheaths have a nice combination of attachment points that can be removed as desired.

My only ongoing worry is that over time removing/inserting the blade will eventually slide the keeper strap, but so far that hasn't been an issue.

Oh, and I like that Doug went with the black nylon. The green nylon sheath that came with my Becker BK10 is a tad too military looking for me.

in my experience the sheath is a decent indicator of the quality of the knife. if the sheath is of high quality, odds are the knife is at least decent, whereas if the sheath looks ready to crumble on the table, odds are the knife itself isn't worth owning.
this doesn't apply to antiques obviously, though an old sheath in good condition would indicate that the knife was well cared for, and therefore less likely to have fatal corrosion under the handle.
none of this is law, but it is worth paying attention to.

Busse makes great knives but sidesteps the issue by not supplying a sheath at all. Personally I find it annoying that a knife costing several hundred bucks ships in a cardboard blade cover, but probably anyone spending that is either going to keep it pristine (ie collector) or commission a custom sheath.

I like Kydex myself. A mixture of Kydex and nylon is really useful. One thin I often do with Mora knives is to stick the cheap but effective plastic sheaths they come with down into a leather holster. You get the beauty and character of a handmade sheath with protection of the plastic one. I will try to take some pics after work.

I recently bought a very nice Case sheath knife that also happened to come with a very nice Case sheath. I paid quite a bit for both, but I wouldn't have sprung for either if they hadn't come as a nice and appropriate package. All good sheaths should have some common features, IMHO. Assuming decent quality finished leather, first on my list is that the sheath part covers most of the knife, including a large portion of the grip, that it have a secure flap cover that keeps the knife from falling out accidentally, that it have a sturdy belt loop, and that it have some kind of liner in the sheath. As long as it has those characteristics, I'm pretty happy.

A few years back I found a very high quality leather holster in a gun-store $5 bin. I tried a couple revolvers and found that it was built for a S&W 686; I didn't have one of those. I bought the holster and a couple months later found a very nice 686 to go in it.

I don't mind paying for a quality sheath.