I got my 135 f/2 purely for photographing pine martens in low light, but I've also used it for near macro work coupled to extension tubes, where you can get some very narrow depth of field effects (which I want to experiment some more with). I've also found myself wanting to do more portrait work since getting it too, simply because of the "look" it gives you. With this lens, it's not just the sharpness, but the whole package it brings you, so it isn't an easy lens to compare to others. For general work though, it probably wouldn't give you anything over the 70-200, it simply isn't quite as versatile.
