I am a fan of both Gaiman and Pratchett and Good Omens was an enjoyable read. My favourite parts were the end notes where the authors discuss working with each other. They inspired me to search for synergy with another. I have to admit though, and this may be unpopular, I found little synergy in the story. If anything it was less funny than books by Pratchett and less weird than books by Gaiman as if they had succeeded in toning each other down, rounding off the edges, until what was produced was more commercial but less uniquely theirs. Reading it, I imagined Aziraphale as Terry Pratchett and Crowley as Gaiman, and those images worked well for me.
Accessible and fun. An enjoyable read and perhaps a good entry point for both authors for a new generation.