Summary
"Shape up" is a short book authored by Ryan Singer from Basecamp on a novel Agile management practice they've found and evolved. It aims to maximise team autonomy with focus on "appetite" instead of "estimation", minimise meetings by "shaping" the feature and introducing an unorthodox 6-week cycle, among other intriguing approaches.
However, a further analysis on the problems that these practices are trying to solve and the deviations from common Scrum guide, helps to understand the pretext they might be a success in Basecamp but not necessary the remaining generic settings. After all, rather than an alternative to Scrum, I picture it as a consolidated version within a specific organization, which provides a valuable case study of adapting Scrum in your own.
Key comparisons with Scrum
Shape
The author puts considerate effort on clarifying the shape stage, which equals to story writing in some way. Although he emphasises not to include too much interaction and technical decisions within the design, he required the challenging balance of pitfall foreseeing, appetite prediction (a variant of estimation even the author might not fancy this interpretation), high level system design; all of which is not obtainable without those decisions, even not openly written. It sounds like upfront design, without passing down the design to the implementation team, and entitle this approach as autonomy.
This approach actually reminds me of typical waterfall outsourcing team style, and might be achievable and most efficient in an organisation with highly knowledgeable product owners and " architects". Meanwhile, the choice not to pass on all the knowledge to the team, appears to allow maximum autonomy of the team, while implies a higher cost than both waterfall and agile team due to its requirement of high skilled roles in all parties.
In most projects I've joined, the challenge is not the autonomy, but the missing of a qualified product owner role at all. On this regard, I find this approach hardly more valuable than existing INVEST principle of story writing.