The worst part is their decision to make this change in the first place, combined with the big hike in pricing. They're lumping two completely different types of customers into one: transactional emails and newsletter/marketing. They are not even close to the same thing, and the vast majority of people using only Mandrill (transactional emails) will probably never use the MailChimp product. If they wanted to, they would have already signed up for MailChimp.

Mandrill used to be pretty cheap and it was free small services that didn't need to send more than 12k emails per month. Now they're forcing everyone to pay in blocks of 25,000 sent emails for $20. That means if I have a very small website and only send out 500 emails per month, I'm paying the same amount as someone who sends 25,000 emails in a month. That amounts to $0.04 and $0.0008 cents per-email respectively, so sending my 500 emails costs 50x more per-email than someone sending 25,000 emails. The pricing system just doesn't make sense and the lack of a free tier is unfortunate.

Another bad part is simply migrating to a new service. Most users can probably migrate in under an hour if they're only using Mandrill as a relay, but there are obviously users who use all of Mandrill's services (i.e. templates) and the migration will take a lot longer.