If you have 100,000 users (which we have), MailChimp is really expensive.

Details: http://mailchimp.com/pricing/high-volume/

Mailchimp is thus $475.00 per 100K per month ($5700 per year!) That is a lot to spend on just email.

With Sendy + SES it costs me $10 to do a send to our 100,000 users.

Now I could play around with switching plans all the time at MailChimp to try and always minimize my costs, but I don't want to play that game. SES + Sendy keeps my costs linear with the number of emails I send, I don't have to play any games with changing plans, and it is significantly cheaper that MailChimp.