With Starter for Ten, I agree that the film ending is a lot better than the book, but there's such personality in the narrative that is so difficult to capture on film.
As for One Day- I devoured it.
Knowing Nicholls from his other works I was expecting the same light, embarrassing humour but actually found this to be so much more. Its not rocket science or a major social comment, but it makes no suggestion that it is. I found it to be a wonderfully in depth study of sparkling characters and their changing friendship. I loved the When Harry Met Sally element, it brings richness to the central relationship and deprives it of being a silly 'love at first sight' rom-com. It was still hugely funny, but didn't have the relentless jokes quality of Starter for Ten or The Understudy.
I also think Nicholls has done a really good job of character development over time. I related really well to Emma's post graduating predicament- and her lack of direction! Its 20 year span gives you a rare perspective on books.
I went to see David speak at a literature festival in Durham last year (he's lovely in person by the way!) and I believe he is working on a screen play. Despite his success with Starter for Ten- I'm very wary as to how this will work out with the characters going from 21 and into their 40s... however I will keep an open mind! All I can say is read the book before the film comes out- its such a joy to read.