The 135mm f/3.5 is a very very good (optically) lens, with unmeasurable distortion, and good resolution, though contrast is lower than some of the other similar lenses.
I love mine, use it for landscape and street scenes all the time, however it wouldn't be my go-to lens for portraiture - a slightly shorter, slightly faster lens might be better. Of course, the 100mm f/2.8 ofers no shallower depth of field than the 135/3.5, so I would consider them both equal for the task, save for the extra working distance the 135mm would enforce. The 135 f/3.5 has great (soft) out of focus rendering in my opinion.
Although the (there was a url link here which no longer exists) lives on my camera (which is actually way to sharp for flattering portraits) I always keep my 135mm f/3.5 close by, it's such a special lens that embodies the "style" of the OM system - well built, small, built-in sliding hood, etc. And really cheap.
If you're on a budget, get one. Otherwise, get the 85/2.0. If money is no object, get the 100/2.0 - that's the OM system's ideal portrait lens (because they unfortunately never produced more than a couple of prototypes of the 85mm f/1.4 GRIN lens - that would have been amazing). The 90/2.0 Macro is too good for portraits
