These are one of those "how did I manage without" products - extremely useful for taking out and about for watercolour sketching on travels or just to have in your car/bag as they negate the need for having both brushes and a source of water - this is both in one. Also ideal for working with watersoluble pencils as they release the water through the capillary action on contact with the paper surface so you don't get any flooding, just a nice steady flow of just the right amount of water. Although the handle can be squeezed if you do want to get some extra water effects, or to clean the brush point. Easy to refill from the tap and very reliable.
Cheaper versions of these do not have the necessary capillary action, are usually tricky to refill, and do not last very well. These are very good value for the amount you will use them!