I can also agree with the most things the others said, she is jugded but her really annyoing past. She is my absolute main and I love this champ.
But I would not say that she is easy to play. Learning her basics does not take much time, but knowing how to play all the possible matchups, how to use your blink/chain-combo effectively (and yes, there are some ways) and especially teamfighting needs a lot of experience and something like a harmony with her kit.
To answer your first question: She is not overpowered, but she will never become weak until they change anything on her. I win almost every laning phase, about 55% of the games since they rebuffed her w missile speed (I already had like 70 games at this point with her) and a 50% winrate in this year. But I rarely lose my laning phase also with other champs (expcept I try a new one, totally fed with Diana in one game xD), so it is not because she is just too strong.
Her lategame is compared to others weak, but if you have 4 items, you will still oneshot every adc and mage (expcept he built roa and other items with tanky stats).
If you like a high mobility champion, have fast reactions and can accept that you can fall behind very hard if you lose your laning phase, then I would recommend her. She will be rarely banned (because it is just WASTED @cheeseburger1837), but can easily be counterpicked which is pretty hard when you do not have 200+ games experience.
You will get problems with almost every champ with cc or range if the player is better than you. Good luck!