well, i beg to differ.
there are PLENTY of comic geeks who have complained about the most minute of changes from comic "canon" to film. go back in time to a comic book message board or comic shop immediately before or after the x-men and spidey movies hit theatres.
rogue's lack of white stripe, rogue's lack of powers, wolverine's height, the x-men lineup itself, spidey's organic webshooters, the green goblin's armor, ben urich working for the ny post instead of the daily bugle, etc etc
i've also seen the LOTR and harry potter movies argued both ways. as being too different from the source material and as being too slavishly devoted.
what it comes down to is context. i've never been a huge fan of the LOTR books. i think that the movies are about as accurate as you can be and in most scenes BETTER than the books. the abscence of all those lame songs and tom bombadil can only be a plus.
i have become very attached to the harry potter books. and the movies so far have been so ridiculously close to the source material its hard to believe. even saying that though, there is so much extra stuff in the books i would love to see on the big screen. and i worry at how they will try to squeeze the next 3 mammoth novels into 3 hours movies. but i know they are doing theyre best to get the most important stuff on the screen. so i deal.
as for the comics, these are completely different beasts than novels. as the others have said. they have over FORTY years of backstory and numerous writers. there have been many character shifts and ret-cons over this time. when making a movie there is NO POSSIBLE WAY to tell everything the exact same way it happeend in the comics. there would be way to much history to explain and crazy plotlines that would lose the average movie-goer.
so the important thing is the concept. the feel. the characters. not the specific storyline details.
the first x-men movie had a team roster that didnt match any roster from any time in the comic's history. some characters were slightly older , younger, less powerful, or physically different than their 4color counterparts. BUT they nailed most of it.
from the opening scene and Magneto's nazi experience.
to rogue's first scene with her hysterical reaction to putting her boyfriend in a coma by kissing him
to marsden's amazing annoyign prep cyclops.
and of course jackman's near-perfect wolverine.
they missed in a few spots (mystique and storm)
but for the most part the movie was pretty true to the characters and the x-men concept.
(mutants fighting to protect the world that hates and fears them)
the spidey movie did the same thing. stayed true to the core concept (with great power comes great responsibility) and the peter parker character (nerd turned hero with the usual teenage personal problems and guilt over his dead uncle)
and x2 continues the trend. doing a good job of fixing what was wrong with the first movie (by giving mystique some actual substance and a brain) . and adding another dead-on portrayal with cumming's nightcrawler, which actually kept his german accent AND surprisingly enough his religious convictions.
these movies also have a pretty good balance of action and character moments just like any comic book. so although you could argue that the comic book movies are very different from the source material. i argue that theyre very much THE SAME.
of course, the batman movies SUCKED because the forgot the basic premise of the post 80s batman which is that bruce wayne is the mask that batman wears.
and smallville has no chance to be great because of the severe limitations placed on it because we know what will happen in the end.
but ive rambled enough after answering your question in the first sentence.
steve