I prefer epub now!
I love 16pt for letter size, but I love 12pt for amount of text per sentence.
Jetbook Color will probably provide me with a sufficiently large text size, while also providing me with sufficient amount of text per page.
Mobi is not bad, just looks simpler than epub. RTF I don't like, because it's filesize is too large. PDF generally is a nono. Even with reflow there are a lot of (especially title and picture) errors.