I use a range of EOS cameras (started with D30, then 10D, now 20D, D60, and 350D) and while the eyepieces have been OK I recently decided to improve the situation.
Most of the problem for me is that I'm left-eye dominant and when I'm shooting vertically (with the camera tilted to the left for the best stability) the eyepiece gave no shielding against bright surroundings to my left.
I experimented with the Canon EP-EX15 which goes between the camera and the eyepiece (Ef on the 3?0D, Eb on the others: the mount is the same). This moves the eyepiece back (with the benefit that your face isn't quite so mashed up against the LCD) and has an optical element that shrinks the image so that if you shoot with glasses your eye can be further back from the viewfinder. That optical element is simply and easily removed (undo two screws and it drops out or back in again) which results in a shielded tunnel to the viewfinder. I found that while I could still see all of the viewfinder it didn't help with the shielding as the eyepiece was still the same.
This week I found what looks like a solution, and (shock, horror) it's made by Nikon.
Nikon make an eyecup for the N6006/N5005 (F-601/F-401X) bodies which happens to also fit the D70/D100 and lots of other bodies with rectangular viewfinders. Unfortunately they seem to be in fairly short supply and the only place to find them is on eBay.
It turns out that this eyecup fits onto the Canon viewfinder. I've been carrying around a 350D or 20D this week with one on and it hasn't fallen off (I figured this would be a good test - seems secure so far).
Here it is on a 350D:
It mounts just as securely on a D60, 10D, and 20D. I suspect it will also fit onto the 1D-series bodies (as they can also use the EP-EX15 with the same size mount) but haven't tested. I have not yet tested it on a 5D: I expect it will fit but I'm not sure if the slightly greater distance to the eye will be a problem for the full-frame view. Certainly an EP-EX15 (modified to just be a tunnel) is a bit "iffy" on a 5D, but this doesn't move your eye back quite as far.
I've found that I can even now shoot with my sunglasses on (the plastic centre of the standard eyepieces are likely to scratch, but this rubber hood seems great).
Just in case anyone else here wanted to try it...
