Another vote for Mandrill - been using it for over 2 years now. I use it on all of my sites, and I manage over a dozen Mandrill accounts on behalf of my clients as well.
I segment my outbound email by various criteria (site, source application, type of email, etc), which makes it easy to identify the source of deliverability issues and identify potential problems with your application.
Can also be especially useful for development servers where you want to capture outbound emails for diagnostic purposes, but not actually send them to a real user.