If you have a few domains and not a ton of requests it might be worth while going with a service but it has been my experience that if you have millions of requests a day and hundreds of domains the price for a service will become ridiculous. Sure it is a big purchase for a couple of GSSs but then you own them. A service might cost less in the short-term but over a year or two you might be paying double what it would have cost to buy the hardware yourself.

At its foundation, it is a DNS server with some special features.

You can get all of this as part of a hosted service where you just pay for the service, no hardware required.