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.