It’s great.
Bringing back Palpatine ties all nine Skywalker saga movies together as one cohesive story and makes The Rise of Skywalker feel like an actual series finale to three trilogies rather than an ending to just one trilogy. He’s so important to the saga, and had he not been included as the final villain, then it’d feel like something’s missing. We have more of an established connection with him than with Plagueis or Snoke or the Vong or even Maul (yes, Lucas planned on including Maul in his sequel trilogy).
It also pays off his desire for immortality in Revenge of the Sith, only for him to suffer in a clone body that is rotting due to him being so shitty at making clones - a fate worse than death.
On that topic, how he survived is explained, and this explanation relies on inference.
It is reinforced throughout the film that he has the ability to transfer his spirit to different bodies upon his killing at the hand of another Force-sensitive, to the point where his main motivation is that he intends to goad Rey into killing him, allowing his spirit to leave his failing body and possess her body if she were to succeed; so, one can infer Palpatine had transferred his spirit to this one specific body shortly after Vader killed his original one in Return of the Jedi.
The body his spirit is possessing is also one he manufactured himself — basically a “clone” body, or at least a body that is designed to resemble his original one, it is clearly established he can create lifeforms, he even created Snoke to train Kylo Ren; his original body was atomized in Return of the Jedi, and it is established he can transfer his spirit to another body after his current one is killed at the hands of another Force-sensitive, not to mention how he explains to Kylo Ren that “[he] has died before…” — confirming his original body did die in Return of the Jedi.
And him surviving does not invalidate Anakin’s arc and sacrifice, because he only did it to save Luke, and even then it allowed him to become the legend he was and to inspire hope across the galaxy by facing down the whole First Order on Crait, hence why the civilians of the galaxy immediately go with Lando to Exegol.
Also, him still being alive did not need any set-up. In the same way, in The Empire Strikes Back it was revealed that Darth Vader was Luke Skywalker’s father and yet it went on to become one of the most beloved reveals in popular culture, despite having no real set-up or foreshadowing up until this point. I sense hypocrisy among those who use this particular criticism.
As a guy who likes and defends both the prequel and sequel trilogies (yes, we exist!), I’m not counting the prequels in this… part, as I am referring to in the context of the original trilogy alone and framing it as though it had just come out in theaters in 1980.
Even then, I am glad there was no set-up or foreshadowing in The Force Awakens or The Last Jedi for Palpatine’s return in The Rise of Skywalker.
The whole point of The Last Jedi is that everything is hopeless with the First Order in charge, until Luke Skywalker — the legend, the man who saw good in Darth Vader and turned him back to the light — shows up to save the day and inspires the galaxy, igniting a spark of hope that the First Order is going to be defeated, one day.
As stated, the scene with Temiri Blagg at the end of The Last Jedi drives this point home; he, like the other children at Canto Bight, are slaves, but, despite that, he still has a sense of hope — their enslavers represent the First Order and how they are in charge of the galaxy, and the children represent the galaxy itself… and us, the audience —, the final shot of the film is literally him looking off to the stars with hope.
If J.J. Abrams or Rian Johnson foreshadowed Palpatine’s return in The Force Awakens or The Last Jedi, it would take that away, because, now, we would be asking ourselves, “Why should we be hopeful that the First Order will be defeated when Palpatine is just going to come back and fuck everything up?”
In fact… the reveal that Palpatine is still alive in The Rise of Skywalker is literally the “I am your father…” of our generation (aside from Thanos’ victory in Avengers: Infinity War), because no one had any reason to believe that one of the greatest and most iconic villains in cinema is still alive — similarly, no one had any reason to believe that Darth Vader was Luke Skywalker’s father after A New Hope first came out. It’s just that the trailers ruined all of this.
Even then, the possibility of his return was (retroactively) set up in the opera scene in Revenge of the Sith, when Anakin asks him if it is possible to “learn this power,” obviously referring to Plagueis’ desire to keep himself alive, and Palpatine responds, “Not from a Jedi,” as he grins at us and a bit of dramatic music begins playing — implying he does know the secret to cheating death.