We handle DNS for thousands of domains for our customers and whilst our existing solution worked it was very messy to maintain and work with so we decided to trial a new solution for our offices to see how it would perform. We wanted something that could be database driven for ease of maintenance and we were personally recommended PowerDNS, so we decided to trial that one first.
For the database we would normally go with MySQL but we wanted an instance of PostgreSQL to play with as we are considering moving our main platform to it at some point in the future.