Not for nothing did A Fire upon the Deep take home a Hugo. There are just so many things about it that are good. The universe within which the story takes place is carefully crafted and very interestingly conceived. Several alien races are presented, two of them in detail, one of which (the Tines) had been very elaborately created. The story is huge and compelling, while the writing draws you onward. And the Usenet-like communications setup is an interesting concept.
Probably one of the things that stands out the most in this book is the structure of the Tines. Vinge does a good job of explaining by showing, and the details of the race were enough to make my head hurt as I imagined their ramifications.
The writing was well-paced. It’s a long book, and some parts felt slow-moving compared to others, but they were never uninteresting. During the more active parts, especially the climax of the book (and the other climax right before it) I was so immersed in it that I couldn’t stop reading. And the image at the end was echoingly haunting.
Simply put, buy this book. It’s eminently worth it.
Slashdot seems to have a review of a recently-released special edition of the book. It has notes by Vinge and those who helped him with his book and is apparently only available in electronic format. If I had a working PDA, I’d consider getting it, since the annotations would be very interesting to read, but I’d much prefer a more open format (mostly so I could read it with Weasel Reader).