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.
The first half of the book is collected published works of his (columns, articles, etc.) including an introduction to P.G. Wodehouse's posthumously published and unfinished Sunset at Blandings. In that introduction he makes it clear how fascinating he thinks it is to see a master at work. I think, based on that, he wouldn't be too upset to see that he got the same treatment.
That said, The Salmon of Doubt is very much unfinished and for me isn't the best part of the collection by far. Still worth a try, even if you just don't read that last section.
I read Patrick O'Brian's posthumous and unfinished book, The Final Unfinished Voyage of Jack Aubrey, and it contained O'Brian's final hand-written notes. Truly from a "master at work".
I was a huge Tolkien fan as a child and tried to read the Silmarillion when I was about 10, having already read LOTR and The Hobbit multiple times by that age. I think I did finish the Silmarillion, but I found it tedious enough that I didn't look at Tolkien again for quite some years.