Wide open the 135L can completely destroy busy backgrounds... and give a very dreamy effect... it just depends upon how close you are to your subject and how far behind your subject the background is.
Stopped down, the 135L gets nose-bleed sharp.
And it's got great, rich color rendition plus a very fast, accurate focusing. It really is a joy to use and highly recommended for portraiture on a FF camera such as yours. Try it, I think you'll like it.
Need a bit longer? The 135L works very well with a 1.4X II or III, too, to give you an "effective 189mm f2.8" (damn close to the 200/2.8 II).
Just got just one! a brand new Canon 135L for the Year of the Horse (2014)