I enjoyed the first three episodes but was less impressed with the last three.
They cut some of my favourite bits - Death and the other bikers of the Apocalypse on the trivia machine. Death in this adaptation wasn't the Death I imagined - he was too small (not just height but personality and presence) and appeared to have had a sense of humour bypass (apart from the "leaving early to avoid the rush" line all of his good one liners were cut). One of the best things about TP's death is his dry sense of humour so, as a result, I found him disappointing. Cutting the other four bikers was a sham too - I love them and their alternative apocalyptic names 
I thought Shadwell was pretty mild compared to the novel. TV Shadwell was laconic and logical whereas the one in the book was manic and, despite his horrible character traits, likeable.
I love Miranda Richardson and thought that her playing Madam Tracy was great casting but inept Shadwell meant that her part was diminished. I did enjoy the séance scene though 
I don't understand why NG spent 20 minutes on the "origin story" of the understanding between A and C. That could have been tied up in a couple of sentences. Crowley was really needy and I didn't see that David Tennant needed to do a Bill Nighy impression. Frankly a laconic David Tennant would have been better.
BUT, I loved Gabriel - he was a fab addition and my favourite line was also "Don't talk to me about the greater good - I'm the Archangel fucking Gabriel"
I've watched it twice now - mainly because it was 15 rated and I wanted to watch before I let DS (11yo) watch it with me. Plus it was a good excuse to watch it twice! DS is going to read the book now - although he says he pretty much knows it word for word from me quoting it at him for the last decade!
I'd be interested in a sequel, particularly if it's centring Gabriel.