My 1st camera is a T2i/550D. The situation: Heavy Heavy rainstorm on a once in a lifetime trip to Cuba. I planned to pull out the Rebel when I was under cover. I covered the camera with the Optech rain cover, the camera was also under a rain coat and an umbrella.
The result was the rain didn't kill the camera but the build-up of humidity did. I believe moisture got in and fried the circuit board.
I upgraded to the 5DM3 with L Lenses for some extra weather resistance. If I'm ever caught in a similar situation, I doubt if I would use the Optech rain cover again. I would protect the camera from the rain but allow it to 'breathe'.