Cassiopeia is one of my favorite champions, and I do quite well with her. She is a bit tricky at first, but can be very strong once you figure her out and learn how to build her in different situations, especially since the 4.21 patch. I generally play her pretty cautiously, since her lack of mobility makes it easy to kill her, but she can pretty easily melt most any targets if you catch them off their guard.
I mostly play her as a mid laner and a support (hello, fellow support main!). I always rush Tear and Rod of Ages, though I prioritize Frostfang and a Sightstone if I am supporting. Rabadon's Deathcap is pretty much essential for her late game build, so that's usually my fifth item. I rarely have cause to build boots other than Sorc Shoes with the Furor enchantment. Apart from the obvious Void Staff and Hourglass, I have also built Spirit Visage, Banshee's Veil, Will of the Ancients, Guardian Angel, and Rylai's Crystal Scepter as situational items. I have yet to try Liandry's Torment on Cassiopeia, but it may do well with her.
I play her pretty much the same in both bot and mid, spending the early game harassing with poison and an occasional Twin Fang to try to zone my opponents away from farm before seeking out fights in the mid and late game. In general, you should maintain good vision of your surroundings and avoid taking risks, as Cassiopeia is unforgiving if you make a mistake or get caught out. Don't be afraid to use your ult to save yourself, especially in the early game. Like any other late game damage dealer, your primary job is to stay alive, so do your best to avoid fights that you are not certain you will win.
I don't know if you would consider Cass viable at your level of play, but I have a rather positive win rate with her at my poor little unranked tier, where games tend to run a little long and any warding places you well ahead. I would recommend her if you like immobile mages (or Urgot), but be aware that she may be heavily changed in the future due to widespread dissatisfaction with her rework (which I largely share, though I did not play her before the rework went live).