There's a fellow on IRC having horrible issues having moved from 14 to 15, doing things I do that work for both of us on 14. He's having issues with NAT and his firewall rules that basically blow up his production.
I suggested he write to freebsd-net@ and I'll link it if he does. His experience added to a couple problems I had installing 15 on what was to be a new production server have taken 15 off my list of planned upgrades for the time being.