Does anobody have any opinions on DVD extras. When DVD first came out, I'd painstakingly go through all the extras, dutifully watching deleted scenes and listening to commentaries by costume designers and teaboys. Nowadays I rarely bother with the extras, and just concentrate on the show. However I would be interested to know if there are any shows that have particulary insightful or funny extras worth watching.
In all honesty, I really can't remember ever particularly having watched extras. A few bits and pieces here and there yes (Fawlty Towers bonus disc, Carry On Christmas interviews), but in general, I just never get around to it.
I don't usually bother with the extras, but the Seinfeld DVDs have some great bonus material. The season seven includes some interesting inside looks, with comments from all involved, and features on Elaine and Larry David's final season. Sein-Imation, classic scenes from the show in cartoon format, is also great, especially The Marine Biologist.
Frasier DVDs are usually good with their look around the sets and interviews.
I really like the commentaries on Spaced. From the point of view of a wannabe like me it's great to hear how their ideas often started as in-jokes and stuff. That said, most commentaries are a little less animated I find.
I really want to see the alternative ending extra when the "Green Wing Special" comes out on DVD.
I watch them now and then. Deleted scenes, inside looks, interviews and some other features - if you care enough for the show to buy the DVD, you most probably will want to learn more about it 
Of course, I've watched all Lord Of The Rings bonus discs, which is no small feat
On the other hand, I don't think I've ever watched any episode or film with a commentary. I tried few times, but I keep getting bored. Stills and pictures, another worthless addition, IMHO. Biographies and discograpies I can get on the web.
To answer you question, regarding britcoms... Fawlty Towers bonus disc is probably the first and only thing that springs to mind.
Not an extra, but... I buy DVDs with subtitles (English) only. No exceptions (OK, very few).
In reply to the earlier comment about Frasier, those DVDs are actually really disappointing. Many of them feature no extras at all and the extras that are there are pretty mediocre. The extras on the The Simpsons DVDs are much more impressive.
Red Dwarf probably has the best extras for any britcom. It has commentaries, documentaries, extended episodes, TV specials, model shots etc.
The Human Remains DVD is also superb as are the I'm Alan Partridge DVDs and the commentary on the first series of Phoenix Nights was very funny.
Basically I think it all comes down to how much you like the show ad how interested you are in the people involved as to whether you will be interested in the extras.
Peep show, each series has more extras than the last. The latest to be released, Series 4 has commentry's over 4/6 episodes, a couple of making of featurettes, deleted scenes, a short documentry about Mark and Jez and an all-time top ten 'peep show' moments. (Still, Series 5 is out in June and will probably have more)
Good thread: just last night I discovered that the final disc in the recently released Hancock box set contains high-quality digital scans (in PDF format) of the scripts for every episode of Hancock's Half Hour (and Hancock). Seems to be the final studio versions too, with camera and lighting directions, production codes, and detailed schedule of the whole recording day, right down to the time of the afternoon tea break! Well worth buying for those alone.
The Dr Who DVDs seem to always have an abundance of extras
I was very impressed with the Darkplace & Man To Man with Dean Learner Dvds. Both sets are packed with hours of great extras (re-watchable extras). I also have to agree that Human Remains has some great extras as well.
Though some of it doesn't make sense, Jam is loaded with great extras as well.
Usually I do watch the extra's that are included on the disc i'm watching. I hardly ever put in the "extra disc". :/
Speaking of extras.
On the Takin' Over The Asylum DVD, it says it has David's audition tape, but I can't flipping find it.
Boo hiss! Could it be an 'egg'?