Progressive lenses are used in the most advanced eyeglasses. They provide a seamless range of near and far focal ranges in one lens. They are relatively new, and are not yet perfected, so there are problems with cost and accessibility that come along with the advances in prescription optics.
Progressive lenses offer users all the full range of vision correction. They do not have to choose between glasses that only help them at one distance or bifocals or trifocals that help users at several distinct distances. Bifocals and trifocals also have visible lines where the lens prescription changes, progressives do not have this. Because of this, they give the best simulation of naturally perfect sight.
Progressive lenses can cause some distortion in the wearers peripheral vision. The lenses are very sensitive to their distance from your eyes and where your eyes are pointed. Because of this, there is a learning curve to the lenses reaching their full potential in helping you. Also, because of these intricacies, progressive lenses cost more to have made than bifocals, trifocals, or conventional glasses.
There are large qualitative differences between different progressive lenses. Although they are expensive, high quality progressive lenses will have less distorted vision.