Here's a link to the Metal Shop pack. Essentially it's most of the models that come in the HD147 head. There's Diezel, Engl, Bogner, Mesa, Marshall, etc.. and a few Line 6 'originals'.
Line 6 - Model Packs
It's more than worth the money to get it. If you click on the link in my sig to my band's website/myspace you can hear stuff that was recorded direct with some of the Metal Shop models.
Honestly, I can't see it replacing a real tube amp head but if you don't have the money or place to use one, the Pod is pretty much the next best thing.