I agree with what's been said above. The dirthole set is an effective cat set, but keep in mind conditions that are dealt with. Asa has a very sandy substrate to trap in which lends itself well to digging a dirthole. In most mountainous country, digging a dirthole would be very difficult. I would only dig one if the ground was soft and a deep dirthole could be achieved, if not, there are some other, very good cat sets.
The walk-thru set probably accounts for the majority of the cats taken by big number cat trappers every season. It's effective and simple.
I personally don't like cubby sets because some cats won't work them. And you completely eliminate the chance of catching coyotes. Most of my sets for cats are well guided, and don't catch coyotes the first time through. The coyote (barring transients) will remember the set and check it out again later. My cat sets remain in place all season and often I will catch the greater percentage of the coyotes that work my cat sets through the course of the season as they will eventually build up the courage to work the heavily guided set to investigate the intriguing odors. If you have a lot of snow to deal with, and don't mind missing the coyotes, than a cubby set is a good option.
The scratch-up set Wackyquacker mentioned is a very good set that I utilize a lot. Scrape up a pile of pine needle or sage duff, stick a couple cat droppings, real cat urine and gland lure on it, bed trap and go. This is very effective on cats, and also takes a lot of coyotes.
I do a lot of preseason work, and will often make my scratch-up sets preseason. Adding the droppings and urine (not gland lure) several months before the season opens. Cats will smell this and in many instances will begin utilizing this location as a toilet. It just about guarantees cats when the season opens around.
Squeakers are a very good option to use as attractants, and they have their place. They can greatly add to the appeal of a set, but there are a new host of factors to contend with when utilizing squeakers.
My personal favorite flagging is a feather or bird wing. I've used tinsil, aluminum foil, CD's, cassette tape 'string', and a myriad of other things, but feathers constantly prove superior. Like Slim Pedersen has said, tease the laziest, fattest house cat you can find with any one of the aforementioned flags and you're unlikely to get much interest. Then do the same with a feather and even the fat old cats will usually show interest. If feathers are legal in your area, than I strongly recommend you use them for your flags. If they're not, fake feathers can be purchased at craft stores. They accomplish the same thing and comply with animal parts laws.
I prefer black feathers over white because the black shows up in snow better, and while to the human eye the white may stand out more (prior to snow) the black is not as easily seen by people and cats can typically detect it just as well.
Hope this helps.