Before investing in a diopter, even if cheap, have your eyes checked. I bought two diopters. One for the F2 and one for the Leica M2 which wasn't cheap. Maybe four months later I found I couldn't see as good distance wise and went to the doc. They changed my glasses and now I can see with the standard diopter in both cameras and have filed away the ones I bought...lessons learned.
Having said that, if you can't get corrected to 20/20 or 20/30 you may indeed need that diopter.