It's everything you need from a registrar & DNS provider with nothing you don't: a thoughtful interface that is responsive even with a large volume of domains, no up-sells, first-class API capabilities (which is great for volume/bulk operations), and the pricing is solid/reasonably low-margin with no gimmicks, no front-running.
Only thing to note is that, at least last I looked, Amazon isn't actually the registrar -- it's some kind of 'affiliate' setup with a 3rd party, but AFAICT it's essentially Amazon.