Also there's additional cost involved in using ELB.
I would prefer having a good dns provider, low ttl, EIP, nginx w/haproxy.
However, the problem with using CNAMEs for LBs is that you can't both host mail and a site at the same subdomain. Ideally, what I would want to do is set up redundant MX records for .pagerduty.com to our mail servers, and also set a CNAME from .pagerduty.com to the ELB for to handle the web traffic. The DNS spec doesn't allow this though (it would be ambiguous).
I've thought about using round robin DNS with a low TTL instead of an ELB. Problem there is you don't get all the fancy auto-scaling stuff. I've also heard rumors that some ISPs have their DNS servers configured to put a floor on the retrieved TTL values...