I'm a Mac user who's perfectly happy with a prs500, thanks to libprs500. There're plenty of multi-format books at Fictionwise and Baen. There's all of Project Gutenberg (including bunches of books pre-formatted here are MobileRead). Given how sucky the pricing is at Sony's store (even after the kindle-related price cuts), who needs 'em? The right pricing for novels is $6.00 quantity one, or $15.00 for six (c.f. Baen and their DRM-free pricing).
I can use libprs500 to convert just about any non-DRM'd book to Sony's format. The only time I ever hooked up my prs500 (or my wife's -- we're a two-reader family!) to a PC was for the initial registration and again for a firmware upgrade.
You can successfully use any of the current crop of readers with a Mac without worry.