I think films based on any book will have problems because every fan will imagine things in different ways. I don't think I've ever seen a film that's matched the picture I've had in my head from reading the book. Although this is the case for the DW films I've seen, I think they maintained the story and feel of the books reasonably well
I'm a fan of Good Omens myself
It was my first Neil Gaiman book too and I can see how the two writing styles blend and I think it works really well
But then, as a fan of TP it's unlikely I wouldn't have enjoyed it!