I had mixed feelings about this one from the second I heard it was to be made because of my enthusiasm for the book. You have to understand what the book means to me both to understand my ambivalence to the idea of creating a TV miniseries and to give context to my review.
I've read a lot of Terry Pratchett's books. At least 24 (I've kept a record of the books I read since ~2005 ... I'm a librarian). The count for Neil Gaiman is similar, but slightly lower. I have issues with both their writing styles ... until they wrote a book together, and it was FANTASTIC. Good Omens is, sadly, their only collaboration. It's screamingly funny - funnier even than Pratchett's best solo stuff, with Gaiman bringing a more cohesive story arc and Pratchett fixing up most of Gaiman's problems with characterization. Sure, the plot is still insane, but the structure, delivery, and humour is magnificent and has made this one of my favourite books (I've read it at least three times). And to add to my doubts about adapting this to TV, I was pretty sure the crazy plot was unfilmable. I mean ... this is about the Anti-Christ who - through a mix-up at the hospital - is growing up in a perfectly normal life in small town Britain as the four horsemen gather and the angel and demon assigned to Earth become best of frenemies over millennia. How the hell do you film that?
When I heard that David Tennant had been chosen as Crowley and Michael Sheen as Aziraphale, I was optimistic. And yet, I didn't like their performances. Aziraphale is ... too fussy. And, devil or not, Crowley's eyes are just off-putting. But the more I think about it the more I realize that they couldn't match the images I'd built up in my head of both Aziraphale and Crowley - largely because those images weren't visual, they consisted almost entirely of personality. Which, inevitably, neither of them could match.
The miniseries is remarkably accurate to the book, with most of the variations I noticed having to do with finer plot points it made sense to cut to shorten run-time. Frances McDormand does the introduction and voice-overs (as "The Voice of God," apparently). The title sequences look like they came straight from Terry Gilliam's Monty Python workshop - which is appropriate. One of the series' biggest plot changes was sending Aziraphale and Crowley back to Heaven and Hell respectively for judgment at the end ... and that was a genius comedic twist worthy of Pratchett and Gaiman that I loved.
In the end I find I still have the mixed feelings that I started with. They stayed true to the book, and they did the best possible job that could be done with such a hard-to-interpret book ... including Sheen and Tennant, who did a good job even if it didn't match my expectations. I'm still not sure they succeeded. I may change my mind if I ever watch it again. I think the book is better, but this is at least fun.