The Metal Pack is TOTALLY worth it. I have 4 basic presets on my XTLive, one being a default chorus patch for cleans that came with the XTLive [minus some drive], 2 of these patches were made from Metal Shop pack amps [Powerball and 5150 II emulations] and one is one that came originally with the XTLive [the Soldano 100 emulation, which is so damn SWEET.] The Metal Shop is worth way more than the 50 bucks it costs, comes with these amp models [this is a .PDF file]
and if you don't like it within 30 days they will totally refund your money.
Sometimes you might land on an amp model and be like 'wtf is this shit' and go back to one of the standard high gain amp models like the 'Insane' or 'Lunatic'. This was how I was the first time I got the Metal Shop pack, but then again I'm as stubborn as they get. I screwed around with it for like a day and out popped my now-mandatory Powerball and 5150 II based patches. Sometimes you just have to stick through it and tweak the fucker out, start with an amps characteristics you like the best and go from there. If you're playing through a cab and poweramp, keep in mind that changing the speaker emulation can sometimes makes a shitty tone sound divine.
There are a few amps in the Metal Shop that I feel could have been done better [or are overall useless to my ears], but in the end this thing basically has been supplying me with nothing but great tones I keep digging up month after month. I highly recommend it. Like I said Line 6 says they'll give you back your $50 if you don't like it. What's nice though is you can pay for it and d/l it right away so you can be trying this stuff out for yourelf in no more than 15 minutes.
As far as I know, it's kinda pointless to hack the XTLive and get the Metal Shop pack that way because the software Line 6 uses to transfer the amps into your XTLive checks for pre-registration on their server and shit like that. There is no 'bundle' or any file on your desktop, like User01 said, that you can just export into it. Besides, it WILL fuck up your warranty.
I heard of someone trying to hack their HD-147 [the amp where the Metal Pack models come from] and instead of adding some more models to their HD-147 it turned their high-gain fire monster of an amp into a fucking Flextone III, which is everything the HD-147 is not. He was screwed, ended up getting his warranty and support privledges voided, and was left with a totally different amp he had in the first place.