The Canon RAF works very well with the 5DI. I'm very nearsighted and remove my glasses to use it, both to get closer to the RAF's image and not to mar my eyeglass lenses. In combination with the viewfinder's focus adjustment it yields a wide diopter range.
At lower mag the entire image is viewable without moving your head. At 2.5x a refocus of the RAF eyepiece is necessary (for my eyes at any rate). I use a Brightscreen on the 5DI -- combined with the RAF it yields a very bright, sharp image.
On the 5DII, however, the most critically that I can focus with the RAF is beaten handily by live view at 10x.