There is an unmatched concordance among the user comments on defining this series "brilliant". It is mitigated by the fact that almost all the comments come from the UK, so let me add two voices not from the British Isles.
My girlfriend and I (she's American, I am Italian) both think that this is not only brilliant, it is really the work of genius. The writers, assisted by an exceptional set of actors, did a fantastic job not only in providing genuine humour at every corner, but in studying costume and society and taking advantage of real situations.
This is not your typical sitcom. It is not an effort of a bunch of people that have to fabricate a show a week forever; it is instead the focused effort of three writers that sat down for months to produce six shows. You can feel this in the perfection and consistence of every detail, from the name of the son (Fernando, from Abba) to the picture of Jet from Gladiators (to host a millennium barn dance at Yeovil aerodrome, properly policed, it must not, repeat not, turn into an all-night rave) that Alan keeps in his room.