I watched the 2 Death Note movies a long while ago (haven't seen the L movie yet), so this is from vague memories...
I didn't really like the first Death Note film much, but I gained a better impression after the 2nd movie. There are definitely differences between the manga and the movies, but somehow it kinda worked? in the 2nd movie anyway.
One thing that I remember for sure is that the actor for Light sucked. He couldn't do Light's evil twisted expressions at all.