I read the Salmon of Doubt, but I honestly don't remember much of it. I do remember at the time thinking that it was interesting to read it, and I don't remember any revulsion or finding it particularly bad, but clearly it was not particularly noteworthy either. Then it was not a finished novel. That said, it was not presented as a finished novel either, and included various other biographical bits and pieces, and as a whole I like that it is out there in the form it was put out, which was more of a tribute to Adams than an attempt to sell another Douglas Adams novel.

On the other hand, though it is of course quite different from a posthumously released book by the author himself, it is quite relevant given that part of the reason Adams wanted to turn Salmon of Doubt into a Hitchhikers book was that Mostly Harmless was quite bleak (I seem to remember claims he intentionally ended it the way he did to prevent being badgered about another sequel):

I quite enjoyed "And Another Thing..." by Eoin Colfer, written as a sixth book in the HHGTG series. I read it with some trepidation, but enjoyed it. It wasn't quite the same of course, but he managed to match the style surprisingly well, and it did give the series a much more upbeat ending.

I think both are examples that it is possible to do post-humous releases both of the authors own work and sequels without it going badly.

I think that very much boils down to marketing and readers, though. If you read either with the expectation that they represent Adams at his prime, then you're set for a disappointment. If you see them as more material for the specially interested, then to me at least they were worthwhile.