Pro: that little green help button.  it may not seem like much but that 1% speed up per a member that presses it ads up fast.

Pro: you can get help with resources when you need them.

Con: the clan battles. If you are below level 15 and are in a clan you lose the protection during the clan and kingdom battles.

Con: some people like to target clans so you may end up more of a target.

Pro: As a clan you are protected by any agreements made with other clans.

Pro: the clan store.  Here you can buy a lot of things that would normally cost gold.  and even better you earn the points to buy by pressing that little green button to help your clan members.  and it cost you nothing. 

Pro: the clan tasks.  You can only get them if you are in a clan.  the extra XP and other rewards are worth it.  and you get the points to buy things in the clan store too.

Pro: its always more fun to make friends and play with them then it is by your self.