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.