During testing for a project that we're working on at the moment we obviously have to go through the rigmarole of supporting the eleventy different versions of IE that the world continues to use.
With the addition of IE11 to the roster, it just makes life that little bit more complex. On the server side in XPages what I'm seeing is that the "isIE" test doesn't work exactly as expected.
If you're testing the simple boolean to see if the browser is IE at all (regardless of version) then you'll be fine (although I'm not sure how that's working), but if you're detecting specific versions of IE, then you'll need to add some manual user agent checks. So in my before render response code I now have something like this:
To detect IE 11 (and later) we have to use the new Trident user agent. And the ideal is that we set the document mode to "edge" which is the default from IE's point of view. You can still revert the document mode to earlier versions if you must but it appears that this is not going to be the case for ever so getting your site working with IE11 will probably be worth the effort in the long run.