The Snow Queen: I first read this way back in high school, and re-read it to refresh my memory for the sequels. This novel is loosely structured around Andersen's tale "The Snow Queen," but it also owes quite a lot to Dune. Like that book, it involves a huge galactic empire far into the future, the domination of corporate interests, a planet deliberately kept technologically ignorant because of its valuable and unique resource, a quasi-mystical group with access to ancient knowledge, and so on. Despite her florid prose, the world Vinge creates is quite fascinating, and many of the secondary characters are intriguing. Unfortunately, the two main characters are awful. Moon is pretty much a Mary Sue, and a lovesick one at that. Her journey takes her through a variety of cultures that are intended to prepare her for her grand destiny, but rather than really take advantage of this, she focuses only on reuniting with her lover, Sparks. And Sparks is horrible -- a whiny little excuse for a man who, when he can't get what he wants, proceeds to commit atrocity after atrocity, all the while never letting up on the self-pity.
World's End: This shorter novel focuses on one of the side characters from The Snow Queen, BZ Gundhalindu. It is a much tighter, more straightforward story of how BZ comes into his own, in a way that ties into the fortunes of the the would-be galactic empire, the Hegemony (and which promises some interesting delvelopments for Tiamat, Moon's home planet). Vinge sure does like to make her characters suffer, however.
The Summer Queen: the direct sequel to The Snow Queen. It's long, very long, and could have benefited from some editing down (losing a couple hundred pages of characters' navel-gazings would have helped). It was also a mixed bag. The world Vinge has created is fascinating, and most of The Summer Queen is taken up with Tiamatans struggling to play catch-up with technology rather than having it develop organically (heh), and the Hegemony racing to recapture lost technology that would reopen the galaxy, two subjects Vinge depicts quite well. And as before, the secondary characters are a good mix of virtues, flaws, skills, and personalities.
The problem is with Moon and Sparks, again. Moon is supposed to be a visionary queen, all but single-handedly merging two vastly different cultures, preventing a genocide, and standing up to the Hegemony, but most scenes involve her moping about because she's delicate and worn out, or dwelling obsessively over her love for a man not her husband. Sparks is flat-out infuriating. He never fully atones or takes responsibility for the atrocities he committed in The Snow Queen, he refuses to acknowledge his own role in the breakdown of his marriage, he selfishly and coldly abandons his own children, and like the big sulky baby that he is, he re-embraces great evil, knowingly and willfully, because his life is not how he wanted it to be. He is a reprehensible character, which would be fine if anyone else at all saw this, and if the text didn't try to make us feel bad for him. Towards the end of the book he makes a big "sacrifice" we are supposed to find touching, but it's really just a way for him to run away from his life.
I think the problem is that Vinge has focused too much on the romantic relationships in the books. That was the problem with The Snow Queen, where Moon's obsession with getting Sparks back seemed downright petty given what was going on; as a result both their characters suffered. And then Vinge decided that the love between Moon and BZ would make a better story, but she couldn't figure out what to do with Sparks. Don't get me wrong -- I love a good romance, whether it is the central plot or just a side event. But because of the huge themes and world-changing events going on in this universe, focusing so much on the love of a few characters, and depicting it as all-consuming, made me want to slap them and tell them to get on with saving the world.
Tangled up in Blue: Like World's End, this benefits from a far shorter page length and a much tighter story; Vinge even toned down on the obsessive love factor! It takes place over a few weeks during The Snow Queen, and involves a police investigation that answers a few lingering questions from the rest of the series. The dramatic irony runs high in this one, so it is better (and worth it) to read it after the other three.