Crowns can be crafted from a number of different materials, each having advantages and disadvantages. These days the most commonly used material is ceramic, however metal or porcelain fused to metal is also used in many cases. All ceramic crowns are favored by dentists due to their natural appearance, strength, ability to color adjust, and ability to resist stains.
Metal crowns are very strong and can last a long time. They are very resistant to chipping or breakage. In addition, sometimes less of the existing tooth needs to be removed with metal crowns versus porcelain or ceramic crowns. While these are advantages, the obvious biggest disadvantage is the un-natural color.
There is just no way to blend the metal color into a mouth full of white teeth. However, while most would not consider metal crowns for the front of their smile, sometimes it is an alternative for out-of-sight teeth in the back of the mouth.
On occasion dentists will use stainless steel for crowns. Stainless steel is generally used for temporary crowns to protect the existing tooth from further damage while a definitive crown is being made, or, more commonly, used for children to cover a baby tooth that will eventually fall out or for adults with special needs where multiple visits are not possible.