Why play Cass?
Now, I know what you're thinking. 'Cass is hard, sure, but she has great dps! One of the highest in the game! You're not gonna get great results from her with just a couple of games of playing, you need to master her in order to unlock her great abilities!' This gets brought up a lot, but is it really true? I mean, if it was simply 'Cass is strong when played well' she'd get picked all the time in pro games.
But she's not.
Her weaknesses are low mobility (having to land a Q in order to get any real escape, and she has to stop to do that) and lack of CC (W is weak at early levels, ult has to wait til six and is conditional.) She does have great damage, though they all require landing skillshots. Her strengths are large amounts of damage.
But that's Syndra's thing, isn't it?
Syndra's simply a better Cass. She has worse dps, yes, but it's still enough to kill a squishy and provide sustained DPS in a fight. She also has additional CC before level 6, and has to aim less abilities. She even gets to cast on the move, while Cass has to stop. Why play Cass when Syndra exists?

and
. And of them I think Karthus is the only one to really match her.
I love Cassiopeia, but she is a hard champion to master. I have had a lot of sucess playing her against champions like Yasuo, due to the fact that her Q can get around his shield. She is squishy, and has low mobility, but the high DPS is a decent tradeoff.