We are disappointed with the lack of SSRS improvements over the last 8 years.  We use data driven reports extensively.  For example, every day at 10am we run some report that emails everyone a list of items in their inbox from a custom application that routes them a task they need to act on.  

The problem we experience is that there is no way to flag an item as "emailed" successfully. So we end up either writing our queries to only select the previous 24 hours worth of data, or we write the select of the data inside an SP that also updates the data to mark them as "emailed" even though if there is a problem rendering the report or a problem contacting the email server, the emails won't actually go out.

Basically if we could have a data driven subscription, and while the server is looping through the data, sending off emailed reports, we could get a hook to update status for the item that was just emailed, or just errored during report rendering.