I'm obliged to say that the Interactive Report Regions is one of the best features in APEX. The possibilities are huge. It's like passing the power to the end users.
A few days ago, I've posted about IRR having an upgrade in the next APEX release. The feature is being called as websheets. The possibility of having a tabular form with inline edit combined with the actual IRR features it's a "just can't wait" functionality.
Still, Interactive Reports are not perfect :) There are some functionalities I miss from the classic reports. I know that I can still use the classic reports, and I do use them many times, but having the possibility I tend to use the IRR as much as I can.
One of the functionalities I miss the most from classic reports is the possibility of being based on a function returning a query. This adds the possibility of a report being dynamically based on different tables or views and having dynamic where clauses. One of the situations where this is very useful is if you want to build a report wizard system in your application.
Also important, would be the possibility to have control over the report template and CSS, something similar to what we have in APEX classic reports.