i noticed you mentioned epub twice, and since no-one had voted for those categories yet i took the liberty of changing them to "adobe epub" and "bn epub".
personally, i prefer strongly to buy drm-free books. however, i also am decidedly weak-willed, and if there is a book i really
really want to read which is only available with drm, provided it's a form of drm i can remove, i will buy it anyway (i try to keep that to minimum, nonetheless, and i remove the drm the instant i download it).
i chose adobe (adept) epub, because epub is my format of choice anyway so at least that way i avoid the extra step of converting the format (and the, ahem, "suprises" that sometimes come along with that). i haven't yet tried BN flavour drm, mainly because the few times i've wanted to buy a book from them, i was unable to due to geographic restrictions.
edit : never mind. i changed my vote to "other" after reading Guns4Hire's post, since that corresponds to my behaviour as well.