I have no complaints with the production quality or the casting of the video series. The visuals are beautiful, and the costumes and special effects bring some of the more exotic descriptions in the book to life. Both Michael Sheen and David Tennant are excellent as the two leads, and I thought the kids playing the Them did a great job also. (OK, maybe one minor complaint, I wasn't terribly impressed with the special effects of the Bentley-on-fire-across-M25 scene. So much more potential there.)
Film adaptation has to strike a balance between getting the details right for fans of the original book, and keeping things simple enough for those unfamiliar with it to understand. Good Omens is somewhat heavy on the side of the details, in my opinion. I think it's difficult to get the very understated humor of the book to come across on screen or in narration without leaving unfamiliar viewers feeling confused or even bored. Plus it would simply take up a whole lot of screen time to fully explain everything so a viewer can reach the same level of understanding that one gets from the book. So the creators of the series decided to stay largely faithful to the book, even if it meant that those who hadn't read the book would be a bit lost at time.
That's not to say that everything is exactly like the book, of course. There are quite a few minor characters and plot points left out. Slightly disappointing to some of us who read the book, but not surprising and not a major problem. I would have loved to see the Other Four Horsemen, for instance, but I can certainly understand why that bit was left out. Not essential to the storyline and would have taken a good amount of screen time.
All told, I loved the Good Omens series as a long-time fan of the book. I'd recommend reading the book first for anyone planning to watch the series, or at least give it a second watch after you've had a chance to get through the book. You'll likely pick up on quite a few things that weren't completely clear to a non-reader viewer.
