I've been using Guitar Rig for a while. The drawback with it is that it requires good hardware on your part to get a decent sound. Going straight into a mediocre soundcard will give you less than stellar sound... as opposed to a PODxt which wouldn't care either way as it can go in via firewire or USB.
Guitar Rig has some very convincing sounds in for certain styles, but I've found more then once when using it in a mix, it's hard to get anything beefy out of it, particularily for high-gain sounds. Speaking of which, it only introduced 'true' high-gain in version 1.2, before then the amps didn't have enough balls on their own to create 'metal' levels of overdrive.
In a mix, Guitar Rig still sounds fairly artificial and 'thin' dare I say. Despite having said all that, it's clearly the superior amp simulation plug-in on the market today.
I'm not sure what the rig was that they had Guitar Rig running on at NAMM, but I'd assume it was high end gear and sometimes that can deceive you into getting something that doesn't sound quite so stellar back home.
