I use an 11x14 for portraits. When everything works, it's fantastic. I was using hotlights for a while, but my models have encouraged a push towards strobe and natural light.
The other issue is cost. I have an 8x10 back and that helps, but each sheet of 11x14 HP5+ is $6.40 at BH (compared to $3.50 per sheet for 8x10).