I love the direct access to our ERP database to enable different types of reporting. For example - I can compare post-sale customer discounts against sales (pulling from the AR and AP sides of our ERP) for live tracking of discounts. Without Jet, this would be a high-effort monthly process. Now, it's a click of a button. Additionally, I can create custom reports to deliver exactly what end users need (and adjust them as processes change) without the risk that a user will accidentally change the wrong filters. Important filters can be hard coded! And more...data warehouses! Consolidating detailed data into flattened structures for easy reporting is fantastic, as is removing historical data from ERPs to keep performance high but still being able to report on historical data.
It is an Excel plugin and so it can crash when doing complex or large reports. It does seem to add a little bit of instability to Excel. This is more true of Office 365 when Office/Excel is being frequently updated.
It's certainly not a big issue but it can be annoying.
We solve budgeting issues by having frequent, automated Jet Reports run in the background and push to synced Dropbox folders. Outsides sales has overnight updates on sales, discounting, promotions, etc. This allows for sales to be more focused on promotions that generate sales. Jet also allows for easy review and cleanup of ERP Master Records - vendors, customers, dimensions, etc. to keep our ERP clean and improve data integrity.