The ease of use, the ability to customize your reports as well as how quickly you can create them! I dislike that there is no way for me personally to add my own custom fields or filters in some cases; however this may be something we are able to work around with future updates from their side.
We have been using it since our first year as an organization, so they've had time to figure out what works best for us (and vice versa). They also offer free training sessions which helps alleviate any concerns about getting started right away without having someone walk through everything step-by-step.