We developed our own solution and bought a whitelist certificate. works pretty well

I'll vouch for Mailgun but only in that I generally host with Rackspace (Mailgun is owned by Rackspace) and Mailgun tends to be cheaper when you host with Rackspace (I forget how many email / month get sent for free).

Mailgun has a nice and simple API but the service is flaky IMO. I get notifications almost daily that their admin panel is having problems; that email delivery is having problems, etc... We recently opened a support ticket because an incredible amount of email we sent (roughly 50k) were bouncing because of an undocumented feature (a perfect storm type scenario).

For ease of use / discounted cause of hosting / low qty send - Mailgun is cool. For anything serious "we rely on email for customer notifications" find someone else.

If I were to leave - I'd go with AWS SES - it's cheaper and IMO, much more reliable but the API is quite a bit more complex and everything is on you - tracking bounces, read receipts, monitoring, etc...

I already worked with Amazon SES, MailJet and Mandrill. But still love Mandrill most.