Quite awhile ago I purchased Cassiopeia and had some decent games with her but was not that enthused about her. I decided to giver her another shot on a whim and man am I glad I did. Cassiopeia is tons of fun and is an easy farmer. Her burst is often underestimated by opponents and a well timed ult can turn the tide of a team fight.
I prefer to play Cass mid and run a 9/21/0 setup in my masteries and run cooldown per levle glyphs, magic pen marks, mana/5 per level seals, and gold/10 quints. I use flash and ignite. I build Rod of the Ages first, then sorcerer's boots, Rabadons Deathcap and Rylais Scepter, after this I may get a Will of the Ancients or Abyssal Scepter or a defensive item like Banshees Veil.
I passively farm with Miasma and last hits thru level 6 staying far back after dropping miasma. This allows me to farm well and make it very hard for enemies to harass or gank me. I am often able to clear minions with miasma and let the minions push into the enemy tower while I stay safely back. This helps me to gain an advantage on them. At level 6 I choose my time to be aggressive and flash in and ult my enemy. I then lay miasma, ignite them and twin fang like a mad man. If I stun the enemy with my ult they are most likely dead even with flash the DOT is often too much with the initial burst.
In team fights stay back and pick the right time to flash in and ult. When done right you can ult 3-5 enemies and often stun 2 or more if not all. It is great if someone can kite the enemy team to a waiting Cass so that all enemies are facing one way. After ulting and throwing down miasma under the group, ignite and twin fang the enemy teams carries. Cass can drop other squishies incredibly fast and a well timed ult can lead to multiple kills. Building some survivability on Cass allows you to choose the time to get in close to the fray and do your damage.
I recommend you all give Cass a try, she is really a great champion and has been added to my rotation. She has great burst and people's lack of familiarity with her often results in them being overly aggressive and out of position. Use this and follow a few of my tips and you should be able to have some great games with her and most importantly a lot of fun.