From what I can tell from it's product page, it seems to have a very gimmicky marketing-speak write up (count the number of times the product is named!). It also doesn't let you run IE7 and IE8 at the same time.
I think it's an example of a good intention on Microsoft's part that hasn't been executed or marketed incredibly well.
Any web developer worth salt has to preview their work on IE6-IE9. And everyone has to jump through hoops to make this work (And no, VMs are not an acceptable solution. Why should I bog down my machine with a VM when I just need an extra freaking browser?)
So there comes Microsoft Expression Web SuperPreview. How did they even come up with such a name? What it does is it allows you to open several frames side-by-side, each with a version of IE. It's a great idea, it's JUST what I needed. But what it doesn't let you do is actually interact with the pages. You can only see, not click. Want to test a JavaScript behavior? Sorry.
The gimmicky name and the lack of marketing evidently obscure it further. No surprise you've never heard of it. I completely accidentally stumbled across it last year.
Edit: Link with screenshots: http://expression.microsoft.com/en-us/dd565874.aspx