I've not used this lens, but if you are looking for one for portraiture a couple of things to consider: -
At f2.8 you will have a very very shallow depth of field and the point you select to focus on will be critical. So do you actually need a lens that fast? Most of my recent portraits in my gallery have been taken with a Canon 24-105 at either f4 or f5.6 and even then the dof is not great.
The other point to consider is the zoom range... ok so with the crop factor, 50mm = 80mm which is often considered the idea lens for portraits but you might want to consider something like the Sigma 24-70 f2.8 which gives bit more reach for close crops, albeit at the expense of the wide angle end.
You'll find review of both these lenses and some of the other alternatives at
http://www.photozone.de/reviews