Shift is based on a really interesting concept. What if you had the power to change your past decisions? How would it change your life? The idea is really well thought out in Kim Curran's novel, and it makes for a thoroughly enjoyable, if not life changing, read.
The main character, Scott, is a likeable everyman, a loser-ish teenage boy with a disfunctional family, who finds that he has this power to alter his past choices, resulting in him being sucked into an exciting new world. Secret government organisations, break-away terrorist cells and beautiful women ensue. It's not exactly original and most of the plot is fairly predictable, but the characters are interesting and the central concept is thought-provoking and well-used.
The best characters are, as in most fiction, the rather ambiguous ones. Zac, teenage leader of the Shifter Liberation Front - narcissistic arse with a God-complex, or genuine freedom fighter? Mr Abbott - kindly history teacher or something more sinister? And what the hell is the grotesquely fat man who seems to enjoy licking teenagers?
Overall Shift is a solid piece of urban fantasy for teens, I read it quickly and enjoyed it. I would recommend it as a light read to people who enjoyed books like Divergent by Veronica Roth or the Cassandra Clare stories.