There is always a concern on the speed and size of XML though people undoubtedly hail XML's interoperatbility. But with these concerns now there is a new and emerging concept called Binary XML (Refer : Fast Infoset Project). If Binary XML develops there is a possibility that interoperability may not be possible. There are also effort to create standardized Binary XML format, possibility of which remains to be seen. I feel that the interoperability is the greatest advantage of XML and must be preserved at any cost. Lets see how the show turns up!