I tend to fall into the camp who champions the idea of databases over spreadsheets, particularly for collaborative applications. However, Excel is an awesome tool that I use frequently. I really like the column filters and expression language (which provided inspiration for Ignition expressions). The simple graphing is pretty slick too. I’m not so strict in my criteria as Simon, but appreciate the spirit. I’ve seen many cases of abusing spreadsheets where a database would be more appropriate. Not a single case comes to mind of someone using a database where a spreadsheet would have been a better tool.
Here are some rough criteria that indicate for me to use a database: when relational “lookups” or aggregate queries are required, conducting concurrent edits especially from different locations, dealing with large datasets, heavy macro use, or any time another application is doing anything to the data. “Large datasets” often refers to 1000 rows, possibly less. Not that Excel can’t handle it, but anything more complicated than finding a single row (like a phone list) becomes unwieldy and starts getting into query territory - databases not spreadsheets.
My personal $.02.