My TH became level 8, and now I can train Hog Rider. Are they good troop? If so, how can I use them effectively?

It's very good troop. I use it to destroy elixir tanks and it is pretty good thief also. Only problems is that it is little too black and costs too much.

heal them

Hog riders are good troops.
You can use them in raiding for huge loot on multiplayer battles or you can you use them in ground attacks during war.
Hog riders are most commonly used in players on town hall level 8 going for a ground attack in clan wars.
In using them effectively, you have to upgrade them to maximize their attack potential.
Train around 26-28 hog riders to bring to war.
Hog riders attack the defensive buildings first.
Hog riders will not attack other troops as long as there are still defensive buildings present so it is important to kill the enemy’s reinforcement troop first.
Drop two of them near the clan castle to lure the troops outside.
Use the poison spell to destroy them and add supporting troops like the archers or wizards to kill them.
Use the heal spell in combination with your hog riders.
The heal spell will help keep the health of your hog riders during the battle.
Drop the heal spell where they are more concentrated to heal as much hog riders as possible.
Do not waste the heal spell.
After a number of defensive buildings are destroyed, you can drop archers or wizards to help destroy other building such as the collectors and storages.