The one thing that comes clear throughout this film is that the Director/writer Edgarn Wright and co-writer/"shaun" Simon Pegg have got to be big zombie film fans. Spoofs done by people who don't fully appreciate their respective genres fall flat on their face (Repossessed, Return of the Living Dead III). In fact, if you removed the comedy aspects of this film and played it straight, it would be a genuinely good zombie flick. The attention to detail is superb, on par with Tom Savini; even the part where David is torn to pieces is reminiscent of "Day of the Dead."
The comedy is subtle in places, plain silly in others. The subtle comedy comes in the form of film edits, mixed with sound effects to make even the most mundane of tasks (going to the toilet, washing hands) fit right into the horror mould. The silly humour would appeal to fans of Big Train, Black Books, even Father Ted.
However, I watched this film with my wife. While I fell about the sofa, almost in tears of laughter, she just watched in amazement that anyone could find this funny at all, so I guess that if you don't have that basic appreciation of serious zombie films, you might simply not get it.
Colin M
<Added>
That should have been "Edgar Wright" not Edgarn - doh!