I don't think many of the mainstream superhero comics are that big of a deal, when it comes to literary content anyway. I look at them more as our version of mythology, childhood stories we take with us when we become adults.
Some were more well-written than others, obviously. Golden age X-men (late 1980s to mid 1990s) were pretty uneven, but having Chris Claremont as your writer does add an element to the comics that makes it more like... well, literature. Alan Davis did wonders with Excalibur when they were around, but that was an Alan Moore creation to begin with, and perhaps the basic potential for decent writing was greater.
Why? Well, most superhero comics exist in this strange nexus of other comicry, to make up a word of my own. Spiderman is a franchise that had four or five different comics starring the same character ongoing at one point, each with a different set of writers and artists.
In other words, yes, they are cash cows. But moreso in the internet era, because it can be so easy to disseminate the images.
Our comic books in 2012 aren't the stuff we grew up with, and the stuff we grew up with was already becoming ultra lucrative. The "traditional" comic ended with the comic book industry crash during the mid-1990s. The existence of Image, and to a lesser extent Valiant and the short-lived Ultraverse, sapped the old structure of whatever it had left.
I'd like to think we can take childhood myth and make it into really awesome stories and movies, but you can't expect Spiderman to be super deep and awesome when the comic itself was so uneven, and even bad, for large stretches.
I guess I'm cynical in that sense, I don't think the original material was all that good for most stuff that's being turned into movies. I feel -some- X-men stuff was pretty good, but if you want to see how uneven the writing talent was, go pick up the re-print of the 1990s Executioner's Song. The X-Factor stuff pretty much blows away everything else out of four different titles that made up the 12 part series. 2-3 years earlier, when Claremont was with X-men, it might have been a different story altogether.
But, I digress. I'm happy to see this stuff made into movies, cuz, I mean, ultimately it was cool to see Thor alive on the silver screen. Warts and all. And if I get to see Thanos in action, even better. The Infinity Gauntlet is my favorite Marvel series of all time.