This is a great question for discussion! And a very timely one given all the summer releases of comic book based movies. I'm a huge comic book fan, and I love that we now have the technology to do the effects well and make them look good on film. Though what ultimately will make or break a comic adaptation is the acting and storytelling, just like in an actual comic it is the characters and story which makes or breaks a book as much as the art.
That said, I react on a case by case basis. I adored the first 2 X-Men movies (though had some nit-picks with Magneto's characterization in the first one) and the first two Spider-Man movies. The third of each completely lost it. I was not too fond of the Fantastic Four flicks, or Daredevil and hated Elektra. The Batman movies have always been fun, even if the first few weren't brilliant. The Dark Knight Returns was utterly amazing. They NAILED the characters, and Maggie G didn't even bother me too much. The newest Superman was dreadful, bad casting, bad story, boring, tedious and kinda stupid.
Thor was an extremely gorgeous movie, and I enjoyed it, especially the cameos by other Avengers, but there wasn't a lot of "there" there. That said, Thor wasn't a comic I ever read much, so I didn't have a clear point of reference.
The first Iron Man movie was glorious, and the acting was spot on, as was the story telling. The second was not nearly as good, but it set up for The Avengers movie well.
I may be in the minority here, but I thoroughly loved both Sin City and Watchmen. Yes, I know there were significant changes to Watchmen, and many purists take issue with that, and I *do* understand. but given the epic scope of the material, Hollywood did a far better job than many expected, and than they could have. The actors nailed the characters, and overall it had the atmosphere, feel, and spirit of the book down perfectly. Sin City was like watching the comic unfold on film, and I loved it!
Jonah Hex had so much potential to be good, and fun, and instead it was a wreckage. If they had gotten Joe Lansdale to write the screenplay, and maybe lost Megan Fox, it would have made a huge difference.
I could probably write on about this for pages, and in tedious detail, so I'll stop now.