Every single one of these movies is an absolute classic. I've been watching It's a Wonderful Life every Christmas for several years now and first saw Rear Window and Harvey five years or so ago. Only very recently did I see Mr. Smith goes to Washington but that's also a fantastic film. If you consider that the recent release of It's a Wonderful Life costs more than this box set, you'll know which is the smart choice.
Anyway I can't comment on the extras as this hasn't actually arrived yet but as far as I'd imagine they will be the standard releases so no real extras to speak of but plenty of language options for those of you who are interested in that kind of thing.
The films as I say are all brilliant, just imdb them for proof.
It's a Wonderful Life 8.7/10 for me THE Christmas movie to watch.
Read Window 8.7/10 (One of Alfred Hitchcock's best films)
Harvey 8.1/10 (All about a crazy but lovely man, another heart warmer)
Mr. Smith Goes to Washington 8.4/10 (proof that political corruption and greed are nothing new, excellently told)
Don't be put off because they are old and black and white (well Rear Window is in colour but besides that), they are all great films that are still relevant today. I bet you can't find another box set full of films rated 8 and above on imdb, especially not at this price. Do yourself a favour and get this now.