The Postmark team writes about transactional email delivery, deliverability, inbound email processing and the infrastructure behind their service.
Commenters recommend it for insight into the variables affecting email delivery, and cite its argument that dedicated IPs are less important than often claimed.
I'm surprised as well, since these services reinforce his point even more. In other words, good reputation = $$$.
Also, I currently use mailgun (and dabbling with mandrill on some new projects) but I'm intrigued by postmarkapp.com take on dedicated IP addresses. Apparently they won't sell dedicated IP addresses because of the time needed to warm them up, so you get a shared IP and their TOS is stricter anti-spam than other ESP's: http://blog.postmarkapp.com/post/14127210172/the-false-promises-of-dedicated-ips
So glad you're happy with Postmark and thanks for the kudos - the entire team really appreciates it.
37S is doing a great job with all of their techniques. Of course, they also have an incredible sending volume on their side that helps improve the impact of any of these techniques.
One of the biggest benefits of our approach at Postmark is that you get those same benefits regardless of your send volume: http://blog.postmarkapp.com/post/14127210172/the-false-promises-of-dedicated-ips
I dropped the idea of using Postmark after I read this blog-post: http://blog.postmarkapp.com/post/1431120311/oddities-in-gmail-delivery-adaptive-filters
In there they admit that mails sent through Postmark still end up in Spam-folders or black holes. That was basically the only added value I'd get from using them.
I'd also recommend subscribing to a dedicated mail service. There are dozens of variables that affect your mail delivery. You should read the postmark blog (http://blog.postmarkapp.com/) for a peek into those.
Some argue that dedicated IPs aren't all that important:
http://blog.postmarkapp.com/post/14127210172/the-false-promises-of-dedicated-ips
Postmark added a pretty powerful editor
http://blog.postmarkapp.com/post/125849089273/special-delivery-postmark-templates