Wikipedia Summary: Set in 2018, the series follows the demon Crowley (David Tennant) and the angel Aziraphale (Michael Sheen), longtime acquaintances who, having grown accustomed to life on Earth as representatives of Heaven and Hell, seek to prevent the coming of the Antichrist and with it Armageddon, the final battle between Heaven and Hell.
There is a certain "Douglas-Adams-ian" style on the series that really made my day (I bet it was all Pratchett fault here, but I haven't read Pratchett yet, so YMMV). And by that I mean the narration throwing you into confusion, text snippets that show up to throw you into confusion and the general "Oh, nuns. But they are SATANIC nuns" out of nowhere. All those misdirections just make the whole thing funnier.
Also, you can see that the whole cast was having a blast doing it -- and I bet Tennant was the one having the best time of all. Jon Hamm, playing Gabriel as a "I don't fucking care about being good" angel is hilarious. Miranda Richardson, whose character later in the series have to share the body with another spirit does an amazing job with the actor playing the other spirit (not going to spoilt everything here) with the mouth motions and personality. Michael McKean making a very strong Scottish accent was also impressive, to the point that it really took me awhile to realize who he was.
So, great plot, lots of fun, actors having fun and doing an amazing job in their roles... Even if the end is a bit obvious, it is a great series. Too bad it is just one season.