I agree that Ahsoka's character growth/arc was handled fine over the course of the series, but I can sympathize with people who were really put off by the "Suddenly, a Wild [Anakin's Padawan] Appears!" nature of her inclusion in the overall mythos.
But even if the decision to write in a Padawan for Anakin that we've just somehow never heard of before is an instance of bad writing, it also a very Star Wars thing to do - let's face it, shoe-horning in random plot elements is almost as common as someone saying "I have a bad feeling about this" throughout the series.
"Luke and Leia's mother dies immediately after giving birth!"
"Wait, the one and only thing we know about her from the original series is that Leia remembers her..."
"Have baby Leia's eyes be open!"
"That's not how..."
"No it's cool, Leia has the Force so it'll work!"
"Leia is Luke's sister!"
"But we just spent two movies setting up a love triangle..."
"She loved him as a brother all along because 'somehow she always knew'!"
"She kissed him full on the mouth in Empire..."
"Darth Vader is Luke's father!"
"But Obi-Wan specifically said Vader 'betrayed and murdered' Luke's father..."
"That's true from a certain point of view"
"That was an oddly specific wording choice if he was trying to be cagey about what really happened"
"Anakin built C-3PO when he was like 9 years old!"
"What? I don't even..."
"It's okay, they'll wipe 3PO's memory at the very end of the last prequel!"
"Then what is the point of including it at all?!?"
Since they did make the call to include a Padawan, I am just happy how it played out in the end, with the resolution of her arc giving one more reason for Anakin to harbor ill will towards the Jedi. At least she was a wedged-in plot point that you can interpret as actually adding something to the motivations and decisions of the characters, and Anakin's fall was one area that really needed some shoring up in RotS.
Basically, I think of Ahsoka as a good "No, I am your father" twist, rather than a bad "I built a droid, wanna see?" twist.
Don't forget Grevious and Dooku's sudden and incredibly debilitating bouts with Alzheimers in RotS.
After at least 5 years of fighting Kenobi, Skywalker and Snips, when they come face to face in Ep3, Grevious acts like he has never seen Skywalker, and Skywalker comments about expecting Grevious to be taller, even though he should KNOW Grevious' height. And then, as Kenobi and Skywalker battle Dooku, the Count acts as if he hasn't seen Kenobi and Skywalker since Geonosis, even though the 3 of them tried to break out of Hondo's compound "3 Amigos" style.
More than Asohka or anything else in terms of plot issues, the Alzheimer effect on Grevious and Dooku really bugs the heck out of me.