I had problems adding a cover image. I add it in the correct place, but when I open the book, there's a large space at the top of the page and the bottom of the image is chopped off.
Also, I like how I can just add the ISBN in Calibre, press a button and have it import the title, author, description, etc., even the book cover (with the push of another button).
I use Calibre to sort my books and import them into my Sony Reader and I like being able to add and search tags and listing the books that are in a series.
I'm using the latest version - 0.2.0