Just picked this up recently, but not started reading this yet. It's supposed to be a collection of stories that HPL either ghost-wrote or collaborated on. If any one has read this, especially people like JD or Gollum or Lobolover, I'd be glad to hear your views on this.
Well.... for one thing, it depends on which edition it is, as the earlier Arkham House edition (and the pb's deriving from it) are based on seriously flawed texts, in some cases with large chunks missing. The more recent (post-1985, iirc) edition (and the tpb deriving from that) has the authoritative texts and more stories (though one, "Four O'Clock", by his wife Sonia, is missing from this one, as the degree of Lovecraft's involvement is generally thought to be quite negligible -- something with which I'm not sure I agree). So the more recent editions would definitely be preferable.
As to the stories in the volume -- a mixed bag. A small number are simply forgettable; one or two are quite awful ("Ashes", for instance, and "The Last Test", which has some very good stuff to it, but is horrendously overlong and, frankly, boring -- even HPL complained that he nearly blew a fuse when revising the thing). Others, however, are true gems, such as "The Mound", "The Night Ocean", "The Curse of Yig", and the pieces he did for Hazel Heald, as well as several others. I even have quite a fondness for the first two ("The Crawling Chaos" and "The Green Meadow"), though I will agree that they aren't necessarily good stories -- they are more in the line of impressionistic mood pieces, and very effective as such, I think (though still, at times, seriously flawed).
Below is a link which (if you're not certain) can help determine which edition it is you have:
The Horror in the Museum and Other Revisions - Wikipedia, the free encyclopedia
Let me know if you have any further questions....