I've read about 5 or 6 of his books, and I think most of them suffer from the same malady. A fascinating premise, a great beginning, and then a slow - or rapid - diminution of direction and plausibility.
In other words what starts out as compelling fiction degenerates into laughable, pulpy, trash.
The last book I read of his, Timeline, is a perfect example of the above.
Jurassic Park is also an example, but it holds together a little better.
The sequel to Jurassic Park (forgot the title already - is it Lost World?) is so bad I threw it down 1/3 of the way through. Really childish.
The best book he's written was his first widely read book: The Andromeda Strain. He should have quit then, but then he would have lost out on a gazillion dollars, because apparently the public (and filmmakers) have an insatiable appetite for his work.