We're pretty heavy senders on Mailgun. I'm not a huge fan of them, in particular now that Sinch seems to be slowly trying to force their hands into everything. Biggest features we like are their analytics and tagging support so we can see open and click rate on the different types of emails we send. Purchase dedicated send IPs is important so our reputation isn't getting tanked by some spammer sharing an IP, and their "Optimize" product which pulls in Google Postmaster, Microsoft SNDS reporting, trap monitoring is really great.