The detailed sales report (found under "Reports" and then "Detailed Sales Report) is a GREAT report. It has tons of detail and can be sliced up in an endless variety of ways. Due to its flexibility, we see it get a lot of use and probably overuse. We say overuse because when someone wants some summarized data it can be an inefficient way to gather it - it captures details of every single transaction line item that matches the filter criteria (typically - all sales between two dates). This can make it slower to run than a lot of other reports if you're wanting to report on a large number of sales.
Our recommendation is that the report not be used if there are more than 500,000 sales transaction line items. If your average transaction line is $15 that equates to $7.5 million dollars in the report. Other reports like the department sales trend report can handle significantly larger sales figures without an issue, though the report does not contain the granularity the detailed sales report does. That said, if you're evaluating multi-millions of dollars of data you likely do not require sales line level detail.