I bought the flip-top case:
which can be seen here and have found it excellent. My only niggle is the securing loop at the top which tends to cast a slight shadow in certain conditions. (It's not really essential - I often tuck it under the top edge of the case).
I find all the control buttons are perfectly accessible, likewise the card slots and the connectors on the bottom edge. No complaints whatsoever. I'm guessing, but I think the case adds perhaps 1/3 of the reader's weight - not enough to give me a problem.
I also have Tuff-Luv's case for the 505 which I think is even better because of the very strong protection in the front cover.