Dragons are mostly used to get Trophies as their high cost doesn't make them great candidates for resource raiding.
Dragons are ultimately useful for Clan Wars. It's efficient to accompany them with Rage Spell, as they will be very effective to get 3 star in single raid.
Dragons are one of the most powerful Troops in the game, and as such it can be difficult to defend against them. Multiple high-level Air Defenses are critical for success against Dragons; make sure that your Air Defenses are behind your primary line of defense so that they cannot be taken out early in the battle.
Dragons are wonderful Clan CastleTroops. As a flying unit, the Dragon is completely immune to attacks from most ground troops, and his splash damage capability makes it especially suited for destroying groups of Barbarians and Goblins. His high health also helps him survive attacks from even multiple Archers or Wizards (although a large group of either will usually be enough to destroy the Dragon).
The Dragon encounters major graphic changes at all 4 levels.
Level 1 Dragons have green-colored skin and black eyes.
At level 2, the Dragon's skin turns from green to purple, matching his image in the Barracks. His eyes also turn the color of elixir.
At level 3, the Dragon's skin changes again to dark grayish-brown.
At level 4, the Dragon's skin turns a deep red; as part of the update announcement, Supercell referred to the new "ancient red dragon." His eyes also start flaming.
The first person to have reached over 4,000 trophies (Jorge Yao) used an all-Dragon army to do so. He used 4 Lightning Spells as support, one to take out Clan Castle troops and the rest to destroy an Air Defense. This was his way to ensure 50% damage practically every time he attacked in order to become the top player and achieve this wonderful feat.
You can have a maximum of 12 Dragons at one time in a complete set of fully upgraded Army Camps. This number increases to 13 if you include the 1 that can fit into a level 3 or higher Clan Castle.
The Dragon's attack was given a splash damage component as of the 12 March 2013 update, making them much more powerful against groups of ground troops.
The Dragon received new art as of the 23 May 2013 update; the new Dragon receives forelegs along with a line of small spikes down its neck and back. This new look is often debated among fellow players to which one looks the 'best', along with the P.E.K.K.A..
The Dragon has a pair of paws and fire in his mouth since the 23 May 2013 update. The appearance has also been changed to a robotic like creature.
A Dragon's shadow is oval, not dragon-shaped.
The Dragon's upon-death skeleton is person-shaped.
The Dragon is one of only two troops to have different appearances at every level (along with the P.E.K.K.A).
The Dragon is one of only two troops of which the picture in the Barracks resembles an upgraded troop, the other being the Wall Breaker.
The Dragon is the only troop with 4 legs. (The Hog Rider technically has 6.)
The Dragon never stops flapping its wings, as it cannot land.
The level 4 Dragon has a very thin see-through line going through the base of the wing. However, this is barely noticeable.
Dragons do not attack Walls, but can deal damage if indirectly attacking. (Clan Castle troops shooting over a Wall, etc.)