Why do I play Cassiopeia? Because she's awesome. She has some of the best lane bullying and damage in the game. Cassiopeia is an AP "mid ranged" caster. She has high sustained damage, like a marksman on crack. She works very well with proc items, such as Liandry's Torment and Rylai's Crystal Scepter. She is not an easy character to play, so I would recommend practicing her in Co-op vs AI games before you play her in normals or ranked.

Yes, no boots. Why? Because buying a sixth item is just plain more fun!

Pros:
Higher AD/AP/Armor/Magic Resist
Total Build Freedom!
Boots don't matter unless you're chasing!

Cons:
Less chasing ability.