I spent 10 years in a scientific job, where we used Excel everyday for many things, mainly data storage and analysis. But I have worked for the last 15 years in Marketing, more specifically the research side of Marketing. My position now is with a major California financial institution as a Market Research Analyst.
As a result, I have grown from a novice user to what people tell me is an expert--their words not mine. The more I know about Excel, the more I realize there is to learn. I know enough Excel to get the job done and/or get myself in trouble. I currently do everything from managing mailing campaigns to building report and data entry systems to demographic and ROI analysis.
I normally have my own work, as well as 2 or 3 small projects for others in the company who want to work more efficiently. I have done work for my own department as well as Accounting, Branch Administration and Insurance Services. And now the word is starting to spread that I know a little about Excel, so my workload is growing even more.
The only limit to Excel is 1) The number of rows and columns and 2) your imagination.