Good:  
1.  Blackpowder firing residue washes/tumbles off easily.
2.  You can store loaded nickled ammo in belt loops without growing that greasy, pesky verdigris.  Great for display cartridges.
3.  In high-pressure rifle/pistol calibers, the case is "slicker" and makes for easier extraction, especially in rough chambers.
4.  Easy identification for "premium" ammo, special loads, etc.
Bad:  
1.  poor quality plating flakes off easily and look bad.  Same thing after several reloads and the nickle plate scratches off.
2.  more expensive.