Cassiopeia is one of the best harassment AP champion in my opinion. She has great farming ability with just her Noxious Blast alone and even better with her that and her Miasma. Great harassment in lane with her Twin Fang. She has amazing team fight support with Petrifying Gaze and all her other AOE abilities.

In the early game she has great harassment in all lanes. She is not that bad of a jungler but seeing the new jungle its not hard to really jungle with anyone minus a few. Mid game is where she starts getting lethal. End game is where she can take on multiple people down if you know how to play her well enough as with most champions.

I play Cassiopeia as a high survival ability Ap nuke. Where you can survive most of the fight in order to do your sustain damage. This build can make you carry some what but doesn't have the high burst as the other builds i have seen.

I use these runes for most of the ap champions i have because you have damage early game which most runes are used for.You can substitute some of the runes for magic penetration runes it doesn't really matter.

Cassiopeia is a very offensive champion so you want to focus on the offensive tree but she is very mana hungry early game even with her passive. So you want that mana regeneration in utility. She is not one of the major burst damage as i have said its very flat damage out put when they are poisoned and being hit by you Twin Fang so that why i don't get executioner.

Pros.

Great harassment
Amazing CC with Ultimate
3 AOEs
Not Easy to Counter

Cons.

Very Little Mobility
Focused in Team Fights
Skill Shot Heavy
Difficult Playing Style