I try to only buy DVD's of movies I think I really need to have, although I've misjudged quite a few. In the case of a movie I'm kind of lukewarm about, like Fast and the Furious, an impeccable presentation can push me over the edge.
My number one consideration is the film itself. I will also consider the quality of the print/sound, as well as extras though, but not nearly as much as the film itself. For instance, if a film like Once Upon A Time In The West comes out on DVD, I'm going to buy it, as long as it has at least a decent quality to it. On the other hand, there are quite a few Criterions that I'm just not interested in buying because I don't care to own the film
I've never understood buying a DVD just because of the transfer. A good package will not convince me to buy a bad movie, but a particularly bad transfer will make me not buy a good movie, like in the case of Ran.
I have in the past bought movies for their presentation (Godzilla for instance) but in the end, I always got rid of them. A DVD has to have a good movie or it won't last in my collection. After collecting since June 1997, I still only have just 65 DVD's.