In our opinion, VERY. There are many factors to consider when dealing with water in and around your chimney, so we always recommend a high quality water repellent.
Repelling the water is much more effective than simply sealing off any potential problem areas. The last thing you want to do is seal water inside your chimney as this could actually cause more harm than good. The most common problem with masonry chimneys is water penetration. A high quality water repellent can prevent all types of water damage, such as staining, loss of insulation value, freeze-thaw damage, deterioration, and ultimately structure failure.
This high grade water repellent will actually allow a deep penetration of the active ingredients. As a result, strong chemical bonds beneath the treated surface allow the product to become a physical part of the structure and provide long lasting water repellent protection.