There is not so much to say about the 135/2, in short it is Canon’s best portrait lens. There are no issues, performances wide open are out-standing, but you still may want to step down since DOF can be too thin on close range head-shoots and on group photos you better step down to get everybody in focus. The 135mm FOV portrait characteristics will give a bit of tele-compression and this effect will be flattering for most faces. Isolation and out of focus characteristics are simply top class, probably beaten only by the crazy expensive 200 f/2.
The design of the 135/2 is very similar to the 200/2.8 and it makes me wonder why the 135 should cost 30% more, they look absolutely identical from material point of view. But Canon pricing is not always logical. Still in terms maximum bang for the buck the 135/2 may well be number 1, specially among Canon's L-glass portfolio.

With 135mm the working distances for head and shoulder portraits are from around 3.5 meters (60cm shoots in landscape orientation) to 10 -12 meters for full body portraits. This can in many cases be a bit far and is actually one of the reasons why I sometimes favor the 85/1.2 over the 135/2 (the other reason would be that the 85 is a better choice for night shoots).

To summarize: the 135/2 is in my oppinion Canon's best portrait lens and the only down side are the relatively long working distances.