They are worth it if you are shooting rangefinders, especially some later Leica cameras which have a dinky viewfinder "picture" by comparison with early M cameras. I find the M8 to be almost intolerable in this regard and had I considered it more closely perhaps would never had bought it for this reason - it makes sure focusing a crap shoot. More over this same issue compounds the difficulty of nailing focus on fast lenses being shot wide open in my experience though I dont know this would necessarily be a problem with better eyes than mine when shooting an M5. (It certainly wasa not when I was younger when shooting an M3).
I use a magnifier from Japan Exposures http://www.japanexposures.com/shop/a...es/viewfinder/
I think the one I have is this specific one http://www.japanexposures.com/shop/a...r-leica-m.html
These have the added advantage of having a built in diopter adjustment that is fully adjustable within the range -3 to +1.5 diopters. A consideration as you get older. But they typically restrict the exit pupil (so you see less of the finder image) as well as the eye relief (so your eye has to be placed quite precisely). If this does not suit you then its a problem. Having said this I do not know of any that are a particular problem with 50mm camera lenses. They tend to be more of a problem seeing frame lines with wide lenses - 28mm in particular.
The big advantage is that by magnifying the split image screen they act as if they increase the rangefinder baseline length which means greater accuracy in focusing.
I could not do without mine. Though bear in mind, my eyes are now officially - crap.