Olympus truly made some awesome lenses with its OM system. I have not encountered a truly bad manual focus prime lens from Olympus. Perhaps that's why OM lenses have a huge following. Another reason is that they made some amazing lenses that even today, are exceptionally good. The OM 21mm f2, 24mm f2, 90mm f2 macro are just a few examples. If you understand that they made some of the best microscopes in its days, and no doubt the expertise was used to create some of these great lenses, then there is not much of a mystery why the lenses are so good. The OM lenses have always been pretty expensive in the used market, and I think it's even more so today.
The OM 35mm f2 is wonderful on the 5D Mark II. It's fast aperture makes the viewfinder bright and easy to focus, its optic is excellent, though on my copy, the far edges are slightly less sharp but still very good. The bokeh is nice, though in some situations, it looks quite distracting.
I think this lens is worth owning. Aside from its excellent optics, it's also very well build, and focuses smoothly. Truly a joy to hold and behold.
Olympus Zuiko 35mm f2 with hood & EOS Adaptaer. Click for larger.
Tail lights from a 1959 Cadillac Fleetwood - Canon 5D Mark II & OM 35mm f2. Click for larger.
Graffiti - Canon 5D Mark II & OM 35mm f2. Click for larger.
Bokeh - Canon 5D Mark II & OM 35mm f2 @ f2
Tree & Fence - Canon 5D Mark II & OM 35mm f2.