The main reason my company uses InsightSquared is for historical/snapshot reporting that SFDC currently simply can't handle. It allows us the ability to see what our pipeline was at a specific point in time.
It is difficult to add and subtract users. I was given the advice to deactivate the user in SFDC and then give them a new license, which seems bizarre. Also, some of the built in reports to show retention, attrition, etc., aren't explained very well, and you have to search in glossaries and help text. That's fine, but not the quickest/simplest way to get that information.
Our instance was set up by a previous employee and my company barely uses it anymore. Make sure to make it as specific to your organization as possible in the implementation phase, as I have struggled to really understand our setup and how to fix it. The customer support team is helpful, but I don't think they want to waste time on essentially a second implementation with us.
Mostly data visualization and historical tracking. As our SFDC Admin, I can admit that Salesforce reporting, especially historical, is nowhere near where it needs to be, and that's where IS has found it's niche. I hope and assume that it is still growing and improving, as it is a good start but needs more to offer to beat out similar, higher end tools.