The studio that I work at has the Nomad Factory Rock Amp Legends plug-in installed on one of our systems. We have a number of different studios on site, and for whatever reason this is only on one of the systems in one of the mixing rooms. Usually we'll have all of the plug-ins installed on all of the systems, but I guess not too many people will use this unless it's during the mixing process. All of our systems are Pro Tools HD based, and the one in particular that it's on is a G5 desktop. Can't say we've ever had an issue running it...
While not my favorite amp modeling software out there, there's still a lot that you can do with the Nomad Factory Rock Amp Legends plug-in. It pretty much has all of the bases covered in terms of different features and effects, but in general I don't find all of the sounds to be totally realistic here. In general I try to get my guitar sounds from the source, but when you need to beef up a sound or do some sound fixing, amp modeling can certainly be helpful. There are a few other pieces of amp modeling software out there that I'll use before this one if I need to, but I've messed around a good amount with this one and it can certainly be useful. I like using it for more subtle changes in sound rather than trying to change the whole tone all the way around. I'd recommend checking out Softube's Amp Room plug-ins and even Digidesign's Eleven before this one, but Nomad Factory's Rock Amp Legends software is cheap enough where there's no harm at all in checking it out for yourself.