Beware! Here lie spoilers for Daredevil the movie, Daredevil the comic (The Murdock Papers), mild spoilers for X-Men 3.
Putting in my five cents:
Comic movies, as a general rule, are awesome. Not only do they expose a wider audience to the joy that is the comic fandom, but they provide different and interesting interpretations of well known and well loved characters.
However, this also leads to the brutal maiming and slaughtering of such loved characters. I'm a Marvel girl - currently avidly reading Daredevil and Fantastic Four - and I can honestly say I was slightly dissapointed with both movies. The casting was wrong for the Storm siblings, for one thing (why cast a Latino girl? Even if she is 'Sexiest Woman 2007', or whatever it was). And I felt so much more could have been done with it.
Now Daredevil. Oh, why, oh why. Brutal maiming. The movie sucked so much, not even the presence of one of my favorite characters could redeem it. Alright, so it could a little. But not much. Reputedly the Director's Cut is better, but I have yet to see it (currently on the hunt for it). The scripting was sloppy, the soundtrack seemed too try-hard rather than cool, and Daredevil's reasoning as to why the Kingpin would keep his identity secret makes no sense whatsoever. So what if it's admitting to getting defeated by a blind man? He had super powers, no shame in that. And Kingpin now has the information to take down Daredevil/Murdock forever (as in 'The Murdock Papers'), and I see absolutely no substantial reason why he shouldn't.
Plus, Foggy came across as slow, greedy and stupid.
The X-Men movies, on the other hand, rocked. Until the third one. The third one sucked. I only wish there was more of Iceman, and that Angel actually did something in the third one, rather than sit around and look... angelic. X3 tried to cram too much into one movie - Jean becoming Pheonix, the uproar around the Cure, introduction of Angel. Too much, too many special effects. It became more of a lights show than anything, though I had a tiny party in my head when Iceman turned to ice for the first time. Again, annoyed that if you blinked, you would miss it (literally, it was on screen for all of 5 seconds).
V for Vendetta also rocked. I was fortunate enough to be told to study it for my Year 12 English course this year (albeit the film version). While there are notable changes, particularly to the character Evey, the film is capable of standing on its own. Multiple deviations are taken from the original comic, so I don't really count them as together - I view them as seperate brilliant works. However, I liked the comic better, for its added depth and complexity, which a movie would never be able to emulate without becoming convoluted.
And: Yeah, orual_dreaming. Constantine sucked big time. Do not watch it, it will destroy your faith in humanity.
And as for film cannonising comics, the only example I can think of off the top of my head is Harley Quinn, the Batman villan, who originated as an original character for the Batman cartoon, but wormed her way into comics and commonly accepted cannon. And I can't say I'm upset about that.