mnathani: I did consider it, but I have no idea how many requests are made now so I have no idea what it would cost me (not to mention some rogue DNS glitch could run up my bill astronomically)
So I stuck with the fixed-cost option, which happens to be cheaper too :p
(I think CF may have a way to find out what kind of request volume I'm seeing, which will be helpful. dns.he.net definitely had no such metrics.)