I've been hunting for a resource like this for years.  I am often away from my desk where I have both a Mac and PC.  I will be using a Mac laptop to make style changes or whatever to my sites and have no clue if I've stumbled on some weirdo bug in IE that makes the site all FUBARed. 

Now I can use this site to look up my site and see what a PC IE user will see.  While I've seen other sites that do this over the years this is the first one to do it instantly (no queue) and reliably.