FYI, I've had great results with the XLS 2003 format. I use both Office 2003 and Office 2007, saved as XLS 2003. I used formulas to create some of the values to be imported, but I always copy and Paste Special as Values to a clean sheet. I use Find/Replace to remove any unwanted characters that might have gotten in from the database export. During an early test of the module, I left a few rows in with missing required DNN fields such as like Email. That will cause failure. So will having duplicate UserNames in the spreadsheet. The import History log in the DNN Scheduler will show any rows that failed. I used Excel to create all the Core and Extended Profile fields (except UserId and I don't map Region). Timezone and PreferredLocale probably should be mapped early on. I didn't and had to update these later.
If your imported users are all from the same area, you can enter a sample and see how DNN actually stores these fields. For example, in the EST part of US, the actual code is -300. The USA English PreferredLocale is stored as en-US.
Starting with clean, complete data in Excel is very important for this module. It did not pick up on defaults I set in DNN, so populating everything and mapping correctly does the trick.