My rating and comment are for the third novel, Fleeting Dreams, only. (Someone really ought to add the first two.) Anyway, each of the six stories have very different tones and need to be evaluated separately.
"Deranged Love" - Gives us a little insight into Rido, but is written in a way that's so vague it mostly just tells us what we already knew.
"Gifts for Yuki" - Typical Yuki, Adiou and Kaname shenanigans. A cute story if you like those three together.
"Hidden Love" - I actually thought this story was a brilliant insight into Sara and her motivations, and this is coming from someone who normally can't stand Sara. Highly recommended since we learn so little about her in the actual manga.
"Queen of the Abyss" - Explores what Zero tells Yuuki towards the end of the manga
about drinking the blood of the vampires he was sent to hunt
. Unfortunately it's done in very poor taste and I found the sexual focus uncomfortably inconsistent with the usual feel of the franchise. I also found it degrading to Zero as a character.
"A Maiden's Melancholy" - This story is told from White Lily's perspective, and while I always wished they would have done more with her instead of dropping her after the first few chapters, I felt like this story didn't fit with the others and added nothing. I'd rather they'd weaved her persepctive into the story we already know, rather than throw out a cheap side story.
"In a Thousand Years" - Finally after all this time Vampire Knight returns to what I always loved about it in the first place: comedy rooted in night class antics. This is what was seriously lacking from the second half of the manga, and while it is a little jarring to return to it after all the melodrama, it feels somehow familiar.
In the end I would say Fleeting Dreams is not nearly as good as Ice Blue Sin, but heads above Noir's Trap. Still, none of the three books was particularly fantastic and I think they all could have been better written as extensions of the plot, rather than tangental fan fodder.