Litmus is the go-to solution!

We are a very small company. I'm really the only user at the moment. I use it for every email that we deploy. I test against a variety of platforms/devices/email clients. From there, I use analytics to track information. We also use Eloqua as our platform, so most of our reporting comes from there.

We don't really use this for informing strategy. It is also tough as the technology for tracking depends upon loading that one-pixel file. We've run into some issues with that in terms of reporting. For example, Litmus reports on the amount of time a user spends on an email. This information is really interesting and can really help gauge how an email is performing. However, Litmus reported that a very high number of our recipients were skim reading (5 seconds or less) our emails. This actually wasn't true, but happened because Litmus is unable to gauge how much time a Gmail user spends with the email.