I use them all the time. I've found it's the easiest, quickest, although dirtiest way to remove rust on a project.
None of them last very long when you use them hard. As said above, the twisted "rope" wire brushes in the heaviest gauge work the best at removing rust and scale.
I find the dewault ones are OK as are the Milwaukee ones.
The cup brushes, although more expensive, seem to last longer than the disc shaped ones, which eventually throw all the wires all over the place.
I actually use a 6 inch cup and disc on a big 7 inch grinder a lot. That extra size and power can do much more work, but be careful, they can be a handful at times.
Don't even think about using one without protection. I wear a full face shield, kevlar gloves, a dust mask and at least long sleeves when I use one.