At AMEE, we've been using Sendgrid (to take care of delivering email) and KissMetrics (to help us understand in detail how our apps are used) for a while now (we'd never be able to deliver the same level of service internally as they can, and they get better for free), but as is often the case when working with disparate third party services, they don't always work together how you want.
One of the reasons we use these two services however, is that they provide a rich enough API to combine them to plug gaps like we mentioned above.
For example, we use Sendgrid for transactional email, and it does a sterling job when it comes to actually making sure email ends up in the right inbox. But if we wanted to see what a user did with an email it had arrived, and log that for analysis later, we could't find a simple way to do this.
Likewise with KissMetrics - it's straightforward enough to create a libary of events in the browser to log, that you can then combine in groups to build reports on how well a certain
