Excel "Unicode Text" == UTF16 Little Endian with BOM.
Other software might generate UTF16 Little Endian without BOM, for example. But the assumed endianness without BOM is Big Endian (speaking generically) so unless Marketo's side always parses as LE, that would be a problem.
It can get very complex, esp. when people have custom export software that could be written in any language. It's good to know that Excel consistently works for you -- anybody with the opportunity to "massage" with Excel should follow your example.
Thanks Sanford, I was getting a little confused.
I have now managed to import my leads using the Unicode option.
Thanks very much everyone, it was my first question to the Marketo Community and your help has made it a good experience