I use both on free tier and I am happier with PostMark.

I had several issues when sending to Microsoft (hotmail, msn, etc.) domains via MailGun because someone exceeded granted quota for outbound IP address and since then I balance outgoing emails using both.

As mentioned by others, MailGun support doesn't care much about queries from free tier users so I would go for PostMark if I had to pay (more intensive use) or recommend to someone.

I also never faced an issue with MailGun, though I mostly use smtp not their http api.

Is there any specific issue you ran into?

For us it takes too long for the messages to be received with mailgun.

You're noticing a delay between mailgun getting the email and it getting to the client?

I'm using their REST api to send emails, and I've yet to see an email take more than a couple of seconds to get to destination (gmail) while testing.

How long is too long?

Why aren’t you happy with MailGun? I’m a customer and I haven’t had any issues with it.

MailGun's support is horrible. I kept emailing them to let them know about login issues, and it would take them a week to get back to me. After a couple tries, they just dropped the ball and stopped following up on the ticket. Message received.