It was excellent. The special effects would have made Tom Savini proud (given the series was made for TV and not the cinema).
The plot was very akin to George A Romero's "Day of the Dead" with the military research bunker being swapped for the BB house and one member of the group lets the zombies in.
Or "Shaun of the Dead" where people are trapped in a pub and one member of the group lets the zombies in.
Or "Dawn of the dead" where people trapped in a shopping mall and a biker gang lets the zombies in.
Or "Land of the Dead" where people are trapped in a city and the zombies let themselves in.
All in all, the zombie genre is pretty formulaic with the driving force being recently reanimated dead wanting to eat the living. So to set it apart any new addition to the genre must be character driven, have an amazing plot twist and pay attention to detail.
Dead Set, in my opinion achieved 2 out of 3, plot twists are very hard to come by in this genre. The strong character narrative was enhanced by famous (and infamous) faces playing themselves. Attention to detail, down to the realistic language; the recognition that people do occasionally need to use the toilet at the most inopportune moment and everybody being panic-stricken throughout most of the story brought the series to life (or living death).
We're all here coz we're not all there