When we go to B&N, we'll usually drop between $200 and $300. The $25 "membership" pays for itself in one purchase, so in my mind, it's a good investment. For others, it may not be so cut and dry- especially if you buy a paperback for $8 and you have to fork over an extra $25 just to save a buck. The membership is very beneficial to the bulk buyers, even if they only purchase once or twice a year.
I've bought books online, usually ones I can't find in the local retailers. The advantage to places like Amazon for me is that they have a wide selection, so when I know exactly what I'm looking for, I can usually find it quick. In general, when I'm book shopping, I don't necessarily have a specific book in mind, and I find it a lot easier to be in "browse mode" in a brick/mortar store than online. You can grab a book off the shelf and flip through it and actually see if it's what you want. With a fiction novel, that may not be as big of a deal, but when I'm buying technical manuals or the like, I'd rather be able to flip through and see what all it has to offer as opposed to the brief blurb that is previewed online, which may or may not be indicative of the quality of the book.