One of my customers has about 15000 email accounts.
I use extensively RBLs to keep the unwanted connections to a minimum, and many other spam detection techniques, many of which involve DNS lookups. Plus all the lookups caused by the email of 15000 users.
It turns out that according to our ISP, we were one of the top ten DNS users in their network, and we were killing their servers.
That got fixed, but it seems lately we were killing our own DNS servers, too.
BIND would stop answering to lookups every once in a while and had to be restarted. Since that was annoying, and the software was up to date, and there seemed to be nothing wrong with the configuration, I did what I could think of, and started looking for an alternative.
PowerDNS's recursor works well, is lighter, it's easier, and works like a charm (but you really should use their multi-forward patch).
So, thumbs up for PowerDNS, and I am still keeping BIND for our authoritative zones, at least for a while, since that part seems to still be working correctly.