I read the book and I very much like what they propose. Is also interesting that they show what parts you can "do without" as a small team.
In my team we have organically come to a very similar process (which we stablished before Shape Up came along).

Here is how we are similar to it, and what I like about Shape Up:

• Defining the problem that we are trying to solve, establishing constraints, and slicing vertically.
• Handing this shaped problem to the team/individual and giving them latitude to design and implement a solution to the problem

Where we are different:

• We work on a Kanban style, so no cycles, iterations, or sprints. So no cool down periods either.
• We keep involved in the design and implementation process after we have shaped the problem, to help the team further refine the problem statement and amend it when new information surfaces