Greg Bear's Vitals is a science fiction thriller with just a hint of horror thrown in for good measure. It starts promisingly, hitting the ground running with Hal Cousins, the main protagonist for most of the book, receiving a strange phone call from his twin brother. This sets the tone for the rest of the book and the tension and paranoia just keep increasing.
Perhaps I am biased (I came to this title already admiring Greg Bear's work - especially blood music and Eon), but I found Vitals to be both a genuine page turner and an intellectually engaging read (quite a rare combination). It is also a very frightening book, in the way that 1984 is frightening, as it deals with issues of identity and freedom of thought. Not since 1984 have I felt the fragility of my own personality so starkly while reading a novel.
Ok, Vitals cannot compete with 1984 for sheer literary power, and it falls short of something like Thomas Disch's Camp Concentration on that score also. But Bear is writing a thriller and as such has slightly different concerns where plot, structure and characterisation are concerned.
As an out and out thriller, the novel stands up extremely well. It manages to create and maintain a sense of fear and paranoia all the while moving along at great pace. Add to that the extremely well handled scientific speculation and you have one hell of a SF thriller.