Internet infrastructure is pretty important, so it's a little more disconcerting to me if some huge percentage of sites are dependent on the same single ISP/datacenter/etc.
sendgrid is like 5 years old: http://sendgrid.com/
mailchimp's version: http://mandrill.com/
rackspace: http://www.mailgun.com/
also: https://postmarkapp.com/
SES is still the cheapest (especially when scaling up) and extremely fast from my experience with it.
Here is a good article I found about SES and competitors such as Mandrill: http://www.ventureharbour.com/transactional-email-service-be...
(First used MailChimp in 2003 and they've always been the bomb.)
The Mandrill service is nice, I agree.
Also, MailChimp has a very cool 404 page: http://kb.mailchimp.com/article/transactional-reports-overvi...