Here's what I started with: I've created an ASP.NET MVC project using default settings. That has produced the basic skeleton with Scripts folder in it. Then I've created a new WebSite, copied the jQuery stuff over from that MVC project, created a new JScript file, included jQuery in it and bang! IntelliSense worked just like that!
And it's not only because the $ object is defined! Far from it! Take a look at the following screenshot:
As you can see the IntelliSense worked not only with the stuff that comes directly from $ object but also worked just fine with my plugin as well!
Here's another example that I've tested - ExtJS:
As you can see the editor was capable of parsing all the sources from ExtJS and provide me with the proper IntelliSense for a an object instantiated from Ext.Window prototype!
To add to all that the editor works like a speeding car - fast like I've never seen it before anywhere else!
I'm sure other IDEs (especially those from Java world) provide good developer experience but VWD 2010 is simply the best!
Thanks Microsoft for making this tool available to everyone!