I am trying the endnote X2 in Vista 64+Word 2007, or in WinXP 64+Word 2003. I believe the lagging issue is not fully fixed yet. If you keep the word open, you can feel the lagging in any other 64-bit programs. It is not the case when the Word is off. for example, IE x64 and firefox x64. It seems that 32-bit programs have much better response now.
The workaround for the previous versions posted on the endnote web site does not help, either.