I work for an ISP. We use it (CARP and relayd) heavily for HTTP and DNS load balancing in an IPv6-laden environment. Performance is better than that we saw a few years ago under Linux.
I did not. I was more familiar with OpenBSD to begin with, and like its slant toward security. Although we do use FreeBSD for a few other servers used as NAS controllers. HAST + ZFS + CARP is great.