i love my 135/f2. i will never part with it.
i agree, on a crop it's quite long (but i note you've got a 5d2 so it doesn't matter), but it does depend on the style. it's a very sharp lens, even at f2. when i take a picture of my little girl i can count her eye lashes at 100%. it's stupidly crisp.
i take most of my photos outdoors and the 135 is perfect for that. in fact, i think it is probably *the* lens for outdoor portrait work... if you're doing indoor or studio work then i do think that, even on a fullframe body it will be too long - you will need quite a bit of space to get a full body shot.
it will be wasted in a studio anyway, part of the magic of the lens is how it turns even the most boring backdrop into a wonderful blur of colour.
personally, i like the working distance of the 135. not too far away, but not in your face. just right for me.
when i get home, i'll post a few of my favourite portraits if you like that i've taken with the lens?