i have one for a 5D mark 1, and i used it when i went australia outfield for a month. basically the pros and cons are:
pros
-improves your grip on the camera
-makes the camera a little 'fatter' cos of the extra layer...feels good to hold it but might be subjective.
-protects against stuff hitting onto the camera (e.g. stuff in your bag, stuff in the car, etc)
-gives you a little more confidence to shoot in bad weather
cons
-the lens armor thing is quite useless (at least on bigger lenses, i used a 24-105mm)
-when water droplets get inbetween the case and the camera, it is SUPER hard to clean it up.
-dust too
-like all silicon cases, they degrade after time and become a little stiff.
-takes a while getting used to the buttons cos they are now covered (you gotta really know what buttons do what)
-hard to change batteries (at least for 5D mk1 it was blocking the battery door)
-maybe a little hard to constantly change lenses as smoothly as without the case
whatever it was, i took it out when i came back to sg and it felt like my 5D became a 500D...felt a lot 'skinnier' LOL!