Almost any lens can be a useful portrait lens; the more important issue is how far from your subject is the camera to be. Distance to the subject determines perspective. Different portrait subjects may be better at somewhat different distances. Most common for portraiture is the range of 4 to 7 feet or so, pleasant for most subjects. Conversational distance. Farther tends to flatten and widen faces; closer tends to emphasize pointy facial features, and ears disappear beyond the "horizon" of the head. People with wider flatter faces would benefit from a relatively close distance to increase the sense of roundness.
The second major issue is how much of the subject you wish to show; field of view, in other words. Say that you've picked 5 feet (1.5m) as your distance... if you want to include only the subject's head you'll need a 135mm lens. With 50mm, you'll get more, of course; a generously framed half-body length. Recently I've been almost exclusively using a 28mm for environmental portraits that include not just the person but his/her work environment such as desk+computer or milling machine, whatever, and getting in pretty close.
So, in a nutshell, I suggest you pick your distance according to the perspectinve you want of the subject, then pick your focal length on the basis of how much of your subject you want to include.
