The results are spectacular. Not just in theory but in practice (full disclosure: I am involved with the Star Swarm demo which makes use of this kind of technology.) While each generation of video card struggles to gain substantial performance over the previous generation, here, the same hardware will suddenly see a doubling of performance.
XBox One is the biggest beneficiary; it effectively gives every Xbox One owner a new GPU that is twice as fast as the old one.
Downside is more heat generation, could be why the Xbox One is so big.
Also improves PC, Tablet and phone. Not sure of double the performance but if you watch the demo's at build there is definitely benefits to be had.
Universal apps will run on PC, Tablet, phone and Xbox One.
Apps (including indie games) coming to Xbox One.
All good stuff.