How secure is Excel's Password Protection?

I recently put alot of financial information in an excel spreadsheet. Now I am wondering how secure the Excel password protection is. Is it encrypted/can it be broken into easily?

I want to carry this on a flash drive on my keychain, so it must be as secure as possible. I will probably put this file into an encrypted disk image (i use OSX) also. Do you think this is secure enough?

The flash drive is appropriate; Excels's password protection can be broken in a flash.