I've been fascinated with these brushes for a while now and so I finally broke down and bought the set of three from Rosemary & Co. in the UK. These are a handmade, sable-blend, brushes designed to enable lining in a more confident and controlled manner than those normally achieved by rigger brushes.
The sizes I picked up are actually the full set (1/4", 3/8", and 1/2") of the Series 770, mostly because I may as well cut my UK to Canada shipping costs. They arrived within a few days, wrapped protectively for travel and in perfect condition. I'm really quite pleased with the service there.
So, in any case, one must try them out! So, I just did some basic lines in sepia with some attempt at back and forth waving (which I think I need to just practice, it looks like some cool effects to be had). You can see here:
As you can see, I'm not sure that there's a lot of advantage in having all three sizes, at least on the surface. Mind you, I'm not a super experienced hand, but I think you can probably achieve all the line thicknesses with just the largest brush. Maybe as I use them more, I'll see a bigger difference.
In any case, thumbs up!