Although it's great to have huge posters of art, and you get to see the detail of the artwork, I honestly prefer 8.5x11 prints. I have my own personal artbook that's a collection of prints I've art traded and/or purchased from artists from cons I attend. I don't have too much wall space to toss a bunch of posters up on, but I really enjoy flipping through my print book.
It's far too awkward to have a poster book. But I can see the appeal that each size provides, so I really appreciate artists that provide art in different sizes. Since I prefer the "smaller" standard print size, there are artists I don't end up buying from because they only sell posters. :\ (of course I doubt this hurts their sells as I'm sure there are like 3 people behind me that would LOVE to buy his/her posters)
I love postcards, mainly because they fit in my laptop case, and I travel a lot, and people are always interested as to what I'm up to. I'm HOPING that I can go to AX this year to get some. Will you be at AX?