Do you have problems with websites not showing correctly in IE8? Do you want to save time by not creating specifically CSS for IE8? Do you have problems with JsIFR3 not showing up correctly in IE8?
If you would answer one of these questions with yes then this is the plugin made for you to solve these problems.
Additionally there is not much that needs to be said about this plugin except from the above. All it does is to put IE8 into the IE7 compatibilty mode which will render websites in IE8 just like they do in IE7. This ensures the correct display of older websites and also saves the time to create a seperate stylesheet for IE8.
Info about the IE7 compatibility mode can be found in the MSDN Blog.