It's easy! The interface of this tool makes it really simple for me as I'm used only with Gmail in my daily work activities (as well as other Google products).
There are some features that could be improved but nothing majorly negative about what we have now so far from our experience using their product since its launch last year until date. We've been able solve all problems related specifically regarding Email Marketing campaigns management through these tools by providing us real time metrics reports which helps improve results at every stage during those engagements/campaigns set up.