Really? I don't think Cass is that hard to use. poke with Qs and when you hit one, run in and murder your E key. Murdering your E key as Cass has this magical side effect where you also happen to murder other things.
Admittedly, CSing with her is difficult because of her slow attack animation, but that's about it. And the ult is a little slow but it's best to wait till they open on you for that.
It's not bad.
That said I also personally prefer Diana.
And I would rather lane against Cass than Diana...
...Yeah, get Diana.