It's an internal mod as it has to be wired to the 'heart' of the amp. It also requires two extra pots to be added for the controls of the power scaling - one for the scaling itself and a master volume to help balance the signal properly. It's quite a labour intensive process and there can be problems with it - it's not a perfect technology.
In my opinion, it's a nice thing to have but it's expensive. You're looking at around £200ish (from memory, don't quote me!) to have it fitted to an amp, plus the amp itself will need to have space inside to have it installed. It works best on low powered amps up to around 50w.
It depends what you want to use the amp for. If you really want control at pretty low volumes, then that's the way to go but you pay for it. If you just want something to take the edge off the volume at gig levels, then a master volume is cheap, easy to fit and it works great. I have just a simple master volume on both my JTM45/100 and my Super Lead Klipp.