I'll admit I'm biased, but I'd highly recommend Postmark if you care about delivery and customer service. I used them for Sifter, and after selling Sifter, I joined Wildbit (makers of Postmark). There are a few options for sending on behalf of your customers with Postmark.
Ultimately, you should check them all out. Try out their APIs, their status pages, their support. No matter what, make sure you test their deliverability. Everybody claims great deliverability, but based on the customers that come to us after trying all of the budget options, a lot of their claims simply aren't true.
The big difference with Postmark is that we don't allow bulk/promotional emails and focus exclusively on transactional email. The benefit of this is that all of the emails through Postmark see higher engagement, and thus we end up with a stellar reputation with the various inbox providers. Gmail even officially recommends separating transactional and bulk email onto different subdomains and IPs so that your bulk email doesn't drag down the reputation of your more critical transactional emails.
That's really just the tip of the iceberg, but from a deliverability standpoint you have to ask why none of the other providers are willing to share their delivery statistics on their status pages. Or they ask you to pay more and upgrade to a dedicated IP.
Another way to gauge any vendor that works great for email is to do some twitter searches. "(vendor) spam", "(vendor) support", etc. You'll get a much more accurate view of how well providers do (or don't) take care of customers. Or you can just try emailing support a technical question to test them out and see who responds the fastest and with the best answer.
Email may look like a commodity on the surface, but I promise there's much more too it than that. I'm more than happy to answer any questions or help out.