Nice start, but I would like a number of additions before I replace my spreadsheet. My spreadsheet collects the same information as Fuelly plus the filling station (P66, breaktime, shell, etc) and the fuel type/octane. This allows additional MPG calculations based on the octane and filling stations used.
If the users can compare gas stations as well as vehicles, it could get really interesting. If a company shows a significant # of users who have reduced mileage at one versus another, the users here might start identifying gas stations or companies who might have bad fuel practices.
One of the ideas behind tagging fuel-ups is the ability to group those with common characteristics. For example, I tag all of my fuel-ups with the name of the gas station so I can view every fuel-up for my Prius at a shell vs. safeway. We have a lot more work to do in visualizing those differences, but you could do the same with any other metric including octane. And we'll continue to work on teasing out those differences.