Here are the basic facts.

Ideal damage 50-
headshot damage 50-
can be used to assassinate
there are many skins for the shovel by reaching levels and goals
To get the silver shovel u need to get 200 kills with melee
To get the gold shovel u need to get 500 kills with melee
To get the diamond shovel you need to get 2 prestige rank.

The shovel is an iconic melee for the game and is a lifesaver. Lets say you have one shot left in your shotgun clip and you miss but there is no plants nearby melee is there to save you, another situation where you would take out the shovel is when you have you target on low health (50 or lower) it is perfect to save ammo. -Strategies- Jumping is so useful, if you are on the high ground jump behind to perform an assassination.

The only time you would use a shovel against shield is when there is NO option, but when you are in the situation make an attempt to assassinate (read Strategies/when to take out ) .