I have a small WWS rain cover I use on a 70-200. It's nice but difficult to use with a zoom lens, they are much better with a prime lens. I also have a lenscoat rain cover for my 200-400 due to those access sleeves, it makes zooming the lens easier. The only problem with the WWS and lenscoat rain covered is the open back. I have thought of using an optech rain sleeve over the camera only to protect the back of the camera but haven't yet done it.
If I know I'm going out in the rain or on a sandy beach I usually use an optech rain sleeve to fully cover the camera/lens and sometimes seal the ends using electrical tape to ensure no water gets on the camera/lens. The only problem I find with the optech rain sleeve is if you put a warm hand inside the rain sleeve it causes condensation on the inside and it onto the camera. Optech rain sleeves make it difficult to turn dials and it's very difficult to put on in a rush (I need to put them on before going out due to the viewfinder and tripod plate, the WWS and lenscoat covers are much easier to put on in a rush due to velco bottom opening). I should trust the lenscoat and WWS rain covers in more than a light shower, I just can't get my head around the open back not protecting the camera well enough. I have tried closing the back around the viewfinder but I then I cant view the LCD, although that shouldn't be too much of a problem.