I recently acquired this and I really like it. I haven't used it with my MAC eyeshadows, but it works GREAT with pigments. I am sure it is possible to mix something up myself using a mixing medium, but this is easy and quick. The brush does stain but that doesn't affect the shadow, as others have pointed out. It is really great for getting some of the trickier pigments that don't last as long to stay on really well.
The only thing I would say is that with pigments, it works best with the finer milled pigments. The ones with courser texture, like for example copper sparkle, are REALLY hard to work with as a liner because it gets clumpy when it mixes with the fluid.
I like it because with my pigment collection, I now have another couple dozen more kinds of liquid liner, so to speak.