The ease of use, ability to track engagement with users as well as content sharing is great! I dislike that it's not available for iOS devices but other than that there are very few issues at this time from my perspective - yet they keep improving things every month so that should be taken into consideration when making your decision too.
Tracking all aspects relating back specifically about our business including what people find useful or helpful regarding various topics we cover related to the services we provide via email marketing etc has been extremely effective overall (we have found over 98% open rates!).