I think the book was better... or to put it more accurately, the story was better as a book... If I were the one to transcribe it to screen media, it would have been a TV series, not a single film... It just drags from one semi-plot point to another, like a lot of those "live the life" movies (Mr. Holland's Opus is another good example)...
Viewing the entire life of a character, regardless of whether the character is human or not, is always done best in series format rather than singular film... For example, Data from Star Trek is very much an Asimov-inspired character (practically the same motivations as Andrew Martin), but you could reveal much more facets of his character by using a TV series... I don't think Data would have had nearly as much depth to his character without all those episodes in Star Trek TNG...
I'm not saying the story was bad... if anything, the story itself was great, and Robin Williams was great as always, but I don't think it made for a good film...