Great topic! Okay, here's my humble take.
From 19 to 22, I bounced around a few schools chasing a graphic design degree. It was only when I took an illustration course did it occur to me that freelance might be an option. Before long I was getting enough work to decide going full time freelance was better than going to science class, and I dropped out. So yeah, self taught on the technique, concept side, but I did learn some valuable lessons about the business side of art while in school (professional practice, invoicing, how to advertise, stuff like that). Now I'm 30, I've got a full time gig at Raven, make comics, still freelance for magazines, and I've been working long enough to have a little perspective on what I've missed and gained by skipping the traditional art school route.
What I've gained::
an outsider's perspective.
Not going the traditional path has insured my work is at least a little different than most because I learned in an unconventional manor. And in entertainment art, of course, it's good to be an individual, to have a look. It's weird, but warts and all, it's mine.
the confidence of ignorance
. Instead of going head to head with the very best at an all star art school, I was drifting in the great plains (Alex can relate to this). Not really knowing how talented everyone was, and how many great artists were vying for the same gigs, allowed me to focus on working and improving, with little time devoted to doubt.
This one's a biggie. It's nice to know I'm not upside down some $120,000.
What I've missed::
Re-inventing the wheel.
Like a tribe-less caveman, I had to figure out the wheel, the secret of fire, and the spear all on my own. It's true that in the age of the internet no man is an island, but we can all admit there is a difference between an internet tutorial, and having the instructor grab your brush and show you what's wrong. I know skipping art school slowed my development.
Contacts and opened doors.
Having classmates that would graduate, spread out across the world, and eventually be in positions to hire and or collaborate with me, would have been really, really nice.
We all have buddies that make art, either in the corporeal world or online. But they are indeed our buddies. At school I would have been competing with friend and foe, and I think that would have given me a little bit more tooth. It also would have helped bone up my "works well with others" skill.
Talking to someone that is living your dream.
This is a biggie. Being taught by someone that is doing what you would like to be doing is a profound experience. A teacher that not only teaches, but works in the industry, can supply one with not only a technical mastery, but practical incite, and even a foot in the door.
Self taught versus degree? I don't think the degree matters, but of course the work does. And good instruction and competition informs one's work and growth as a creative. When I look at where I'm at, I wonder if I've arrived here because or despite skipping school.
Keep in mind not all art schools are created equal, and neither are the instructors. If you can find a good school, and more importantly a great instructor, than going to art school is worth the debt. You will grow. But if you aren't in a position to attend the very best, you might be better served striving for improvement on your own. Well, as alone as you can be on an artist's forum.