I generally watch a lot of movies and then read the book, not the other way round as Iknow the book is gonna be so much better.
I totally agree with LadyCaitriona about 'Watchers'. Dean R Koontz is one of my all-time favorite writers and I actually saw the film first before reading the book. On it's own it is a good film for it's time (I was a big Corey Haim fan) but then I read the book...What a let-down !! They tried to copy the book more when they released the film 'Watchers II', but again it fell short. At least that time they actually had a ex-green beret.
Another film that was a let down was 'The Beach'. Again, the film came first for me and I liked it. Reading the book brought some scenes more to life but it also showed how much more was missing from the film. I couldn't put the book down as it takes you on a rollercoaster ride.
Here was something funny, I read the book first before I went to see 'Star Trek: Generations' and there was so much missing from the film that was in the book, worst was that some of the jokes were lefted in the film without the information to make any sense of them. An example of this was when 'Geordie La Forge' was captured by the Klingon sisters. He was tortured and injected with nano technology into his eyes. You never saw this in the film, but when he was released he made a one liner joke about it. Now unless you read what had happened to him in the book....you would have no idea what he is on about.
In general....A Book opens up your imagination and paints imgaes for you instead of a film thrusting it upon your senses. Most films you do not need to think what is happening whereas a book, you need to remember what you have read.