I wear glasses. I used to just push them up on my head while peeping the viewfinder, but now I'm a bit older and can't focus without them. I just push the viewfinder right up to my glasses and everything looks better, but I have to look around inside the viewfinder to see everything because my eye is at least a half-inch farther back than it should be. Also, I'm limited to my old pair of glasses for this because I don't want to scratch up the lenses on my new glasses. And I only expect it to get worse over the years.
The built-in dioptric adjustment on most DSLRs is adequate for now. For my old K bodies, I have a right angle converter. It corrects my vision problem and even magnifies the viewfinder, but it is obnoxious to use AND its a mirror image. It also doesn't fit on old M bodies, so now I have an excuse to get rid of those.
What's a GOOD solution? I know there are several doodads out there that claim they will solve all my problems. I bought the right-angle converter. Its a solution, but not really because I don't end up using it. Is anybody using a dioptric correction add-on that works like a charm? Either film or digital.
How about contact lens wearers? I don't think I have heard anyone complain about leaving a contact lens on the viewfinder. It would be a drastic change, but then I could buy cheap sunglasses.