Metallic, or carbon metallic brake pads as they are commonly referred to, have some advantages over ceramic pads, but those advantages may not be outweighed by the disadvantages. The disadvantages may all be addressed by the ceramic pads. Metallic pads have excellent heat characteristics, but tend to have other problems that make them less desirable than the ceramic version of the same pads.
Metallic pads are noisy. Brake noise can be traced back to a harmonic vibration caused by movement of the pads due to rotor run out, and the amount of play in the brake mounting hardware. Ceramic pads, while still affected by this vibration, raise the harmonic noise out of the range of the human ear. It's still there, but we just can't hear it anymore. The softness of the pad also helps absorb vibration much like the old organic compounds. Think of the difference in the sound of a quarter spinning on a table and a rubber washer spinning on the table.
Metallic pads eat up rotors. The problem here is caused by the fact that metallic pads are harder than the other pad compounds. When the pad comes into contact with the rotor, the rotor will wear down along with the pads. Ceramic pads are very soft, and only the pads wear. Because they are also more heat tolerant, they will not wear as fast as the metallic pad compounds.
Dust build up on your wheels is caused by the high carbon and metal content of the metallic pad. Because these materials hold static electricity, they are also drawn to the metal in the wheel and brake system, resulting in thick, black build-up. Ceramic pad compound have no magnetic properties to draw the dust to the wheel, so it is less likely to stick. The dust is also a light grey colour instead of black. Any dust that does make it onto the wheel will be less visible.
Where the metallic pad shines is in its broad heat range. It functions properly at low temperatures as well as very high temperatures. Metallic pads do tend to hold heat, and brake fade due to fluid boiling still happens. The ceramic pads' only weakness is low temperature efficiency. They need to be warmed a little during the first few stops. After warm up, they work well. And they don't hold heat, so they recover from a hard stop much faster, preventing fluid boiling and brake fade.
The bottom line is that metallic pads work well and do stop better than the semi-metallic pads. However, there are some drawbacks, and these are addressed very nicely by ceramic pads. The downside is the cost. According to AutoZone's website, semi-metallic front brake pads for a 1995 Mustang cost £12.90. The ceramic pads cost £7 more, at £20.70. However, the price is offset by the extra life expectancy of the pads, and they come with a lifetime warranty.