At a glance
Although Man Plus uses many classical SF elements such as travelling to and colonizing Mars, an apocalyptic world view, and the technology-driven transformation of a human into a cyborg body, the actual core of the book is the exploration of what happens in a person's mind if that person becomes disconnected from his or her human surroundings and even his or her own human nature.
This unusual focus on psychology, combined with an interesting narration technique, makes Man Plus quite a special story. Because of this, and because it is simply a good and entertaining read, I can wholeheartedly recommend this book. If you already know and like other works by Frederik Pohl, reading Man Plus is a must. If you belong to the minority of the SF reading world population who has not yet made contact with Pohl's books - why not start with this one?
At a glance
Singularity Sky is a straight technology-oriented story which starts out grippingly, but then, after the initial third or so, loses its initial captivating power. The book dazzles with many good ideas, but story and character development cannot keep up and somehow get lost in the "information overflow". Although the story cursorily explores a few moral and social questions, no new insights can be gained from reading the book.
I don't recommend reading Singularity Sky, but if you can't resist the lure, try to focus on the firework of ideas to avoid disappointment. If you have gone this far, It might also be a good idea to read the follow-up novel Iron Sunrise to tie up any loose ends (of which there certainly are many).