I switched my MVP to AWS SES yesterday after having used Mandrill for transactional emails for about a year. I almost decided to just pay the $20/mo for a MailChimp plan because of SES's documentation and setup complexity and the annoyance of the sandbox (you can only send mail to and from pre-verified email addresses while in the sandbox).

However, I filed the support ticket to get out of the sandbox, requesting 2.5K daily emails. They approved it several hours later with 50K daily emails (!), so they are quite generous there - I was worried I'd have to keep an eye on the number of emails I'm sending, but the amount they gave me gives a nice buffer.

edit: Just to add, I'm one of those people that would religiously open spam emails and look for the relay, and if they're reputable like Mandrill or SendGrid, I'd go to their abuse page and report the email by pasting the email headers. Extrapolating from how many spam mailing lists I got added onto by recruiters, I can see how MailChimp employees' time could get sucked up investigating a lot of these spammers who probably slummed off the free or low cost tiers, and could damage their bounce rates.

So I can't totally blame them for this move w.r.t. Mandrill, and I still plan on using MailChimp when I have a need to send marketing emails to my users.