I use Fictionwise when the price is right, Baen of course (I just this AM decided to buy the whole Jim Baen's Universe), and a couple other places.
Format does not matter to me except that I will pass on any book which comes in only PDF format. And that is for the obvious reasons of formatting on a small display. As in anything under 17" displays.
I almost never visit the used book store anymore as so many books can be had, with some patience for sales from Fictionwise or elsewhere, for just a tad more then the used book price. Used PB's around here run about 50% of cover price. So between $3-$5 ea. but one cannot shop for them in the comfort of one's favorite hole-y underware and torn tee-shirt.
Still there are some, er...ummm...OK....MOST, reads which cannot be found as ebooks. I have made the decision to simply wait and hope for those and am exclusively going with ebooks from now on. I feel that is the only way to push/force publishers to come towards the light.
In all honesty between my 10-year's worth of NYT Xword puzzles and just my current ebook backlog, I would never need buy any more books or d/l another xword puzzle in my lifetime. But, where is the fun in that?
Oh, I have at minimum 500 ebooks right now. Read about 150-200 of them so far when I find the time.