Cassiopeia worth RP?

I don't see Cass being used often, though I always knew that she's a very rewarding character with a steep difficulty curve. I was watching a stream where she was used really well and pretty much sold me on her.

I went over my budget on Christmas shopping and my bank account is going to be light for a long time, so I'm kind of hesitant to spend any more. But the +300 RP for purchases right now would allow me to get more bang for the buck, and it would seem like a pretty big waste to let that pass on top of the free 450 RP that I just got.

What do?