Their support basically refused to take my money and switched us to a different sending server that day and our emails were no longer hitting the junk folder within possibly less than two hours of first contact with their support.
As a developer it's incredibly easy to integrate with, the dashboards are very helpful for debugging, and the one time in at least a few years I've reached out to their support they were informative and expedient.
I've found their competitors to be harder to integrate with and focused more on mass mailing for marketing, wanting to funnel everything through their UI.
I hope Mailgun can keep up all the good work they're doing, it's a breath of fresh air to know sending emails from our web applications will take all of five minutes to integrate.
I literally went through the same problem, and they did the same thing for me, they didn't accept money for a dedicated IP
It seems that spammers are using their free tier to send spam emails, and because of that, some of their non-dedicated IP range are listed as spam by Office365/google
We did eventually get on a solid IP and after I spent several days calling every little local ISP that was flagging our emails based on razor2 our last deleiverbility issues went away.
One thing I'd love to see is a direct integration of mail events with Slack. Currently we pass the events through ourselves via a webhook.