What are your experiences with unofficial sequels to novels?

By unofficial, I mean a sequel/continuation of a novel, but which was carried on by a different author.
(if there is a specific term for this, please inform me)
If it is not a direct sequel, then it is related to the original work, in some way or another.

Here is an example, the novel Scarlett by Alexandra Ripley, which is a sequel to Gone with the Wind. I read the original book as a teen, and while I found it a bit hard to read, due to writing style and the culture included within, I still thought it was a fun book. I just started reading Scarlett, didn’t even know that existed until recently. Not very far into it at all, so there is not much I can say, plus I am reading a translated version so I should probably leave my opinion out of the style, and writing. I’m enjoying it so far though, but yeah, not even 100 pages in.

Another example is Mina The Dracula Story Continues by Elaine Bergstrom. Personally I did not enjoy this at all, because the author turned the protagonist, Mina, into a weakling. (and her husband, Harker as well) I loved her in the original Dracula novel because she was strong and smart, but in this sequel she does not do the character any justice. Just falling for another vampire again, eh. What a slut. What was interesting though, was a back story on one of the three succubi from the original book. That was cool.

So, just small examples from me. I am aware of the general response to these two books, but we have our opinions. And as I say, am barely anywhere in Scarlett yet.

But enough o’ me ramblin’, what are your experiences with unofficial sequels? What were they, what were they about, and did you like them? Hate them? Did they ruin the original story for you? Or improve it? Tell us how.

My only request is that your choices are books that are related to original works, but which were not written by the same author. Thanks.