I submit to you that yes, yes they are. The first time I read (as a kid) one of those Scholastic Comics (I think that's what they were called, they're collector's items, now) versions of 'Treasure Island', I knew I was reading literature.
Comics did go through kind of a Dark Ages, where they were just pulp, pap, and popcorn for the mind. That idiot Stan Lee was to blame, and the art was clunky for both Marvel and DC, and comics became mind-candy for little kids who didn't know any better, and adults spurned them.
Then came the new generation of writers, and actual artists (not just cartoonists) that told their stories in picture. Many split off from the big companies and formed their own small ones. Quality improved, both of writing, art, and the books themselves stood out because they began to use quality paper and have them printed beautifully.
And individual artists began to stand out, and create their own mark. Todd McFarlane (Spawn). Frank Miller (Sin City, '300'). And writers like Garth Ennis began to push the envelope out beyond, far beyond the boundaries of good taste and decorum. See: Preacher.
Taboos were met head on, and broken, nay, shattered, and the 'Story Arc' was born. There had been feeble stabs at it, but mostly to keep characters in the buyers mind, and sell tie-in products. Now, each comic became a chapter in a book, that would end, often with that particular character dying.
And adults started buying them. Slowly, at first, but then the floodgates broke. I had outgrown comics years before, but I suddenly found myself swept up again, and I routinely see adults of all ages in my comic shop picking up their orders. Oh, you still have the little kids coming in with their Moms, running to the bottom racks and picking out Donald Duck and Richie Rich...I think Nattie's in love with Ritchie Rich.
And wanting to know what he is saying has driven her to learn how to read. Bonus.
If you asked me how you could get started, like I did, I would just have to shake my head sadly. Those comics are now worth ridiculous amounts of money. I own comics that could easily bring in $500 or more. I watch it happen on eBay all the time.
They reissue Frank Miller TPB's (Trade Paperbacks) all the time, and you could do worse than start there. Find yourself a decent comic shop, run by a serious comic geek, and ask him (I've only know one geek chick, ever) where to start.
I recommend getting your feet wet with the first arc of Preacher. And any Frank Miller, but the geek can get you the books in order that you need.
Happy hunting, and bon appetite.
Oh, and if you're into horror, and also comedy horror, check out Steve (30 Days Of Night, not a comedy) Niles. And he works with some fantastically odd artists. I love his work.
I just picked up this afternoon the first two episodes of the new 'Serenity' series. I haven't had a chance to read them yet, but the cover art is absolutely superb.
Check it out.