yes, binary format is more efficient. it saves the data as xml format, more quicker to access the the files on opening and less chances of corruption.

Excel Binary Workbook (.xlsb) :As Excel Macro-enabled Workbook, but storing information in binary form rather than XML documents for opening and saving documents more quickly and efficiently. Intended especially for very large documents with tens of thousands of rows, and/or several hundreds of columns.

for more info
http://en.wikipedia.org/wiki/Microso...ML_Spreadsheet

all the best