Format matters to me because
1) I am Lazy
(note to ProDigit - I bought a Cowon PMP solely because it supports more formats and I am too lazy to convert my music)
Seriously, I don't mind converting an occasional book but if I need to go out of my way often for anything, I find things get put aside once the novelty dies.
I bought my Kindle because
1) Content pricing/availability
2) Whispernet
3) Price (I got in at the $100 Chase offer)
Ok, so I'm lazy AND cheap...probably in that order