I do, indeed, use slave DNS servers, in fact, I’m currently in the market for a second independent provider.
What features am I looking for? Honestly, a competitive amount of POPs and ability to accept AXFR in. I don’t need much more than that.
Oh and pricing: I’m looking for something on the level of AWS or cheaper. I’ve tried approaching some other players in the field like ns1 and Hurricane Electric’s commercial service and those are quoting me $350+/month for < 100 zones and <10m req/month. No thank you.
I certainly don’t need anything as robust as 1 per 1000km. I currently utilize ClouDNS as my main slave DNS provider. ClouDNS give me POPs in the capital city of every economically-relevant country.
I don’t necessarily need something that robust for a backup slave provider. Something like 2 POPs per continent would be more than enough, say South Africa, North Africa, Sydney, Singapore, 1-2 in Europe, 1 in JP/KR, 2 in USA, and one in South America.
That should give decent-enough coverage.