I've tried just about every substrate over the years - and sphagnum moss works well, but becomes a pain if you're keeping a lot (10+) snakes - I always come back to aspen or paper.
I like it for the fact that (in my opinion at least) it is a good mimic to a corn's natural habitat - even more so when you use the American sphagnum (like Wisconsin bog with all the sticks and stems, as opposed to the soft "clean" feathery New Zealand sphagnum), and it looks decent too. With a thick enough layer, corns will tunnel, dig, and just have a good ol' time. When they turn blue, mist half of it down with water to moisten, and the shed will come off clean.
Like Robbie said though, too much moisture is not good, but with enough airflow (screened lid or a tub with a lot of holes) sphagnum moss dries out faster than you may think. Despite that notion, you should never add so much water that the sphagnum moss is "dripping wet" (I mean you're not raising pacman frogs right?).
The only problem I really noticed was it's hard to spot clean. Crap isn't always easily noticeable (like with paper and aspen), unless you shred (pull apart more) the sphagnum moss. The problem then is you tend to have this dusty mess. So if you leave it more unpulled apart, its hard to spot clean and you may end up going through a lot more aspen. Yet, you can reuse sphagnum moss by giving it a good rinse and letting it dry - you should still replace it at some point.
And if you ever consider setting up a "living" vivarium, there are plenty of places in NC to harvest fresh sphagnum.