- It is in Ubers, one of the few non-mascot-legendary Pokemon to be banned from general playing.
- It has a great movepool with both Priority as well as boosting to plain good moves like Sacred Sword / Iron Head / King's Shield etc.
- It can work multiple roles, as in Physical Sweeping, Special Sweeping or even Mixed.
- It can turn the tide of the battle by itself. A lot of Pokemon probably can but Aegisash deserves a special mention imo
- Its ability is astounding and used properly, boosts Aegislash's stats to 60/150/150/150/150/60, making it a terrifying force to be reckoned with.
There are some cons, however:
- It must be properly EV Trained. You should know exactly what it is that you want it to do, and train it so.
- It requires a good deal of practice to know when and how to use Stance Change to your advantage. Switching stances at the wrong time can ruin your game for you.
- Splashing any four random moves from its arsenal does not make it great, what it needs is a proper role or direction. King's Shield is a must have, in 99% of the cases.
- It lacks access to reliable recovery
- It is weak to Earthquake, arguably the most used move in the game
- It is slow, so without priority, you are forced to use strategy over sheer power to out play your opponent.
- King's Shield does not protect it from status, and it is crippled by both Burn as well as Paralysis.
To conclude, Aegislash is like a double edged sword (haha) that is very effective, but only if you know how to use it to your advantage. While it can win battles by itself, it requires practice and planning to be as effective as possible. Should you choose to use it, you should focus on strategy rather than trying to brute force your way to victory. Otherwise, a faster sweeper will use Earthquake or Shadow Ball when you are in Swords Stance and your Aegislash will be KOed before you know it.
A couple of movesets I personally used to great success were:
King's Shield / Swords Dance / Shadow Sneak / Iron Head or Sacred Sword
King's Shield / Substitute / Flash Cannon / Shadow Ball
The first is a simple approach, but the second requires a good deal of guessing and timing to keep your Sub up, specially as it lacks access to recovery.
Final warning: The reason Aegislash is so good is also why it is sometimes hard to use: Due to Stance Change, your Defenses and Offenses are interchanged, so all the EVs in that stat will prove to be of use in the other as well. With respect to the two sets I provided, the first needs EVs in Defense to help in Attack, whereas the second is benefited massively by Special Defense EVs which is converted to Special Attack on attacking.
Other sets to use can include the use of Gyro Ball (0 Speed IVs and Speed hindering nature required for maximum damage)