It only lets me launch IE 5.5 and IE 7 instances on 64-bit Vista. It's also not independent from existing IE settings (from registry), so if your IE is locked down, this will be locked down too.
It would nice to run each version of IE with application-level virtualization, where each has its own fake registry to mess with.
I just run a copy of XP inside VirtualBox to test things in IE. It's not application level virtualization, but I definitely don't care what happens to the registry, as I can just swap in a fresh disk image whenever I want.
I love VirtualBox, but it's takes up a lot of space to keep several XP images on my notebook. The fact that this lets you test in IE5.5 through IE8 on a single image is a nice win, IMHO.
It would nice to run each version of IE with application-level virtualization, where each has its own fake registry to mess with.