I believe it to be a real collaboration, where the question is almost meaningless. There are some points where we can clearly read Pratchett's humour and some places where a bit Neil's darker imagining shines through, but on the whole I think it is almost impossible to separate one from the other.

It will however remain on of my favourite books of all time. I can also recommend the TV series made a few years ago. It was probably as true to to book as you can make it and had me laughing out loud almost as much as the book did.