I'd say a native excel format is way too complicated for the device. Even the .xslx format which is XML-based requires the XML to be compressed. It would also require you to write the data together as one file - whereas a delimited file can just be appended when you have new information. However - if you use a .csv file extension and comma separate - excel can open the file without user interaction. CSV files are typically already associated with excel - so they would open it up like a normal spreadsheet.
