My main requirements were;
1) Shows up as a mass storage device in whatever Operating system is on the computer I happen to be using at that time.
2) GSM/3G since when I travel I'm often nowhere near any wi-fi.
3) Recognizes standard formats (mainly epub)
4) Expandable storage
I ended up getting the nook which had the added advantage I could look it over in a store and make certain The screen was comfortable to read and the unit felt sturdy.
So far I've mostly bought a few short stories and downloaded epubs from project Gutenberg. It's all worked rather well. I think as time goes on I'll probably mostly use it for periodicals. Once I've read the Public domain books I'm interested in any purchased reading will probably still be 90% paperbacks. Unless a fair bit of new stuff becomes available in Public domain or really neat things get released in digital form only.
I mostly read for pleasure and generally buy paperbacks, often used, so the $9.99-$12.99 e-book price (U.S.) all the online stores keep jabbering about is way more expensive than the $2-$8 I generally pay for a shaved tree book. But magazines and newspapers that I read are actually cheaper on e-readers than in physical form. Much nicer getting the occasional copy of the FT for $0.75 instead of $2.50