Its not fair to compare mailchimp to amazon ses, mailchimp is a full newsletter sending platform that handles everything. if you need only email sending there is 100k mails for 20usd with sendgrid.
Sendgrid also does marketing emails now right? Your thoughts on it?
You'll ned a dedicated IP as their free plans use shared IPs which are often flagged as spam sources.
Source: Tried the free plan a few times.
If you use GAE, you get a free plan on SendGrid (up to a max) and I've used it to send mails to gmail and yahoo accounts. Don't recollect anyone ending up in spam but I haven't sent them in thousands. Will keep your comment in mind as I do so..
If your sending volumes are low a dedicated IP can be worse than shared IPs.