Mine finally came today for my PRS-350, I got it for $37.99 and free shipping from
B&H Photo. It adds about an inch to the width of the reader, so it's not bad at all.
In addition to what others have already said about it, the front and back flaps of it have a slight magnetic pull to stay closed and attached to the reader, which I like. You can bend the top all the way back so you're not holding an open book (unless you prefer to hold it in the open position). I really like it so far.