As has been said - it is one of the best 135's you can find. Resolution is already high at f4 and all you really gain by stopping down is depth of field. The lenshead also makes a great "macro" lens on a Visoflex. You need the extension tube for it but those are not that difficult to find.
The new Apo is probably marginally higher contrast, but I doubt you will see much difference between them. Cheap too - unless we keep hyping it up here!
The Elmarit 135f2.8 is OK, but can suffer dislocated focussing prisms in the goggles. This one becomes a really long rangefinder lens on the M8 (about 175mm f2.8!!). Good quality lens, but a bit clumsy to use. The Tele Elmar 135f4 is better!