Well new to Office 2007 anyway.

I posted this question in General Questions on 29th March - no reply as yet.

If binary is more efficient speed and space-wise, why isn't it the default
format?

I'm trying to find out what the downside is before I save a majorly
important, large file with heaps of macros in binary. If it isn't the
default there must be a reason - a down side - but I can't find anything in
Help to tell me.

Anyone know?

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