I'm not exactly a spreadsheet maven but I do use Excel occasionally and, usually, have a hard time with it. It works but it's not the easiest application to get along with. Since I will probably upgrade to iWork '08 for the new Keynote I thought it would be a good idea to try Numbers as well.
I installed the 30-day trial (not sure why they seem intent on hiding that link, I couldn't find it anywhere on the iWork site) and fired Numbers up. I started a new sheet based on the budget template. Immediately you see how Numbers shines over Excel. Instead of a big grid of cells you have a nicely laid out set of tables that present the structural content of your spreadsheet in a way that is both friendly and easy to manipulate.
The formulas in the various tables are all interlinked and you can add new rows to the tables just by dragging them (or using the add above/below context menu items). The pop-ups (for selecting months in this case) work nicely. In fact the whole thing works really nicely and I was very quickly able to build a nice budget spreadsheet like I never quite managed to in Excel.
I've heard that Numbers performs poorly on even moderately sized data sets that Excel has no problem with. It's a 1.0 product so I guess I don't find that too surprising and expect it will improve. In terms of usability though I think Numbers kicks Excel's ass. If it weren't for the Ubiquity of Office I could see a lot of people using Numbers. It's a pity for them that they probably won't.