It is reliable and costs $1 per 10,000 emails sent.
As for where to go -- Sendgrid and Sparkpost are popular offerings in the same space:
http://blog.mandrill.com/important-changes-to-mandrill.html
https://sendgrid.com/blog/how-to-migrate-from-mandrill-to-sendgrid/
https://www.sparkpost.com/migration-guides/mandrill/
Also I mean "absorption" because at some point Mailchimp required Mandrill users to start using Mailchimp credentials to log in. Even though they were the same company ultimately, the transition felt like an extremely sloppy acquisition!
One potential pitfall is keeping your blocked-email list synced between ESPs. You definitely don't want to repeatedly send to an address that has bounced or reported your email as spam, as that's a good way to get yourself into an ISP's spam filter for all its recipients.
When using multiple ESPs, the best approach is maintaining and enforcing your own block list. But if you're relying on your ESPs' own block list functionality to prevent additional sends to invalid/complaining addresses, you'll either need to sync their block lists, or somehow partition things so that the same recipient email always goes through the same ESP.
Nearly all transactional ESPs offer webhook notifications on bounces and complaints, but not all have an API where you can add an email to their internal block list.
If this had been a simple mistake, fine (all ESPs have occasional issues), but to my knowledge SendGrid has never acknowledged this as an operational error. In fact, at the time SendGrid representatives repeatedly insisted the change was made intentionally for its (paying) customers' benefit.
(That said, SendGrid had some really nice tracking and segmentation dashboards, which I do miss at my new ESP.)
Currently on Postmark, and have found them to be reliable, transparent about the very rare problem on their end, and helpful with problems on my end.
Following Mandrill’s policy changes, I started maintaining a package that simplifies moving Django projects between a number of full-featured transactional ESPs. Even if you’re not on Django, you may find the list of ESPs it supports helpful. [2]
That said, we have a duplicate of Mandrill’s sending infrastructure at SES and use it enough that we know it is ready for migration. We have a lot of domains (agency.) Keeping track of domain sending security for two providers is occasionally challenging, but we could move everything to solely use SES in about a week if needed.
Has anyone migrated off of Mandrill in the past few years? Where’d you go, and how was the transition?