I want to be sure all my Google Spreadsheet work can be converted OK to Excel. How many of the Excel formulas match the formulas of Google spreadsheets?
Almost all formulas are same in Microsoft Excel and Google Spreadsheet.
There are both pros and cons when it comes to other features. So make sure before you start migrating, you need to sure to what extent you need to use the Google Docs.
Here are few links that will help you in understanding pros and cons.
If you're only worried about formulas, it shouldn't be a big problem, as Google Spreadsheets is now mature enough to support all the typical spreadsheet functions. It might be that the names are not exactly the same, or that some of them have to be done in a different way. So unless you're using some obscure stuff, you should be okay. I suggest you get a list of all the functions being used on your Excel Spreadsheets and then try them in Google Docs.
Also, if you're using any VB macros, you can forget about Google Docs.