She's often referred to as an "ADC-mage" due to her mechanical intensity. She was a favorite of scripters (much like 5.0 AS Kog) as when played perfectly she can weave AA's in and put out insurmountable numbers.
However, this is not the case for 99% of players (myself included). Cass has absolutely crushing DPS and sustain long-term if she's able to keep a target Poisoned and spamming E on them. She is very immobile (without summoner's and accurate Q's) as a tradeoff, and her W having a minimum cast range makes it that much harder to keep sticky targets off of you. Her teamfighting potential and Ultimate are second to none, check out Faker playing Cassiopeia in NA (mid fekar) and look at the intensity she requires and delivers when played well.
Note: Not trying to advertise for these YouTubers/Streamers, just wanted to show HQ videos and perspectives for you:
https://www.youtube.com/watch?v=Ggn1yAxezwk
https://www.youtube.com/watch?v=I6f9i6TCW1s
A fed Cass is nothing to scoff at, as she will, pretty much, 1v5 an entire team with her kiting - or at the very least bring multiple members to their knees and force incredible amounts of pressure. She's one of the few Champions in the League that can actually stand toe to toe with Ryze.