I can NEVER watch a movie that was 'loosely' based on a book if I've read the book first. Movies always pale in comparison.
You are right, even though I try to see films and books as separate art forms, the book is almost always better than the film with the exception of the works of some very poor writers like Alistair Maclean and much of the production of Stephen King.
Sometimes it can be intersting to see how a film maker has dealt with the story and how they have visualised certain things. I found for example The Unbearable Lightness of Being
a wonderful film, because of the cast and the acting (Binoche and Olin in particular) even though I had read the book beforehand.