At the 40k+ volume levels, you're also going to get a complimentary dedicated IP, which means better deliverability. You can get that at Mailgun too, but it's a $59 surcharge every month.
Finally, the InboxTrail comparison posted elsewhere in this thread shows Mailgun getting flagged as spam 37.5% of the time.
Our support team is here to help 24/7. We have a great team here that is proud of the work we do for our customers.
Disclosure: I lead product development at Mailgun.
As a Mailgun user, I can confirm that first hand.
It's true we offer a premium product, but we do so at a competitive price. At scale, our pricing model compares favorably with Sendgrid. Our pricing strategy is all about simplicity and providing lower prices as you grow with us. We don't have any kind of feature gating pushing you into higher tiers nor do we have punitive overages.