I loved Neuromancer and was happy to find that it was made into a sort of trilogy, but I was quite disappointed with Count Zero. I think it is a combination of the writing style, characters, and overall plot.
As part of a trilogy, it does not have all that much in common with Neuromancer other than the world it is set in. None of the main characters from Neuromancer appear except for the Finn but it's only a cameo appearance here. We get the impression that the Wintermute AI sort of split into multiple entities at some time between the stories, which is suggested to be a few years.
As for the characters, none of them really appealed to me the way the ones from Neuromancer did. The main protaganists are underdeveloped and rather bland at the end. They just weren't that sympathetic and I couldn't really get myself to care about them.
Then there is the writing style. While Neuromancer was written entirely from Case's point of view, Count Zero is seen through the eyes of three different people who take different paths throughout the story. At the end of the book the paths converge but they do so in a rather sudden and Deus-Ex-Machina like way that is hard to swallow. It felt to me like Gibson was running out of pages and had realized that he needed to tie all these plot threads together. The book could have used a couple more chapters to straighten everything out, rather than having the non-ending it has like Neal Stephenson's Snow Crash (a small flaw in an otherwise phenomenal book.) This writing style however, has become common in Gibson's later novels, though fortunately in Virtual Light he learns to tie the three characters together better and in Idoru he sticks to only two main protagonists, which makes it easier to follow.
Overall, I would only recommend reading Count Zero if you intend to read Mona Lisa Overdrive (final book in the trilogy) as it takes off shortly from the end of Count Zero with some of the same main characters and developes them more. In the big picture, Count Zero doesn't stand very well on it's own and mainly bridges the gap between the beginning and the end.