The Valdemar Companion has many details that would excite any fans of Misty. From the new short story about an "ordinary" herald, interviews with Mercedes and the editor Betsy, a discussion of how the works become music, a description of the online Valdemar where you can become a Herald, the "traveler's guide" where you get a better picture of the landscape, the summaries of all the Velgarth books up to Take a Thief, a discography of all the albums, and a concordance of virtually every character in her books, up to Take a Thief.
Even with all this wonderful information, I wanted more... Other than the wonderful cover art of Vanyel, the only artwork in the Valdemar Companion was the detailed maps of Velgarth. This from the series that would have a picture of the characters every chapter. I wanted more on the myths and legends of Velgarth and how it shaped that world. Perhaps even descriptions of all the different beings in Velgarth (with pictures) like the Companions, gryphons, kree, firecats, bondbirds, etc. Ha! Even a list of Shin'a'in proverbs! In short, I wanted more details of Velgarth, how it was put together, to learn all the details that makes it unique that you can't glean from the novels.
For those who have not read all her books, there will be spoilers. And for those of us who have read all her books, it's nice to have as a reference as you are rereading her books. Though, is it worth the price of a hardback? I suppose it depends on how great your need to have a complete collection...
All in all, I would have preferred if the Companion came out a little later (maybe after Exile's Honor so it could have included any info from that), with more thought on what the readers want.