Theturnaround is absolutely right, it was a choice of aesthetics. The film dictated the mediums used. Its not always about getting the best image, but getting the best type of image to tell your story. I thought it was a great choice, and a courageous one at that. He had a lot of people going against him on that miniDV move, but he stuck to his guns. I know a lot of people who love the look of 8mm, they don't pick it apart because of the lack of resolution, but embrace it for it's look. I love cinematography and crave the highest resolution i can get, but its important to keep in mind what its all about in the end, telling a story.
I saw the movie in theatres. I was a bit disappointed with the miniDV in the beginning with some of the wide london landscape shots, but as the story took off, i really got a feel for what Boyle was trying to do. I thought it was great.
;)