Zach Epstein at BGR gives us a good, yet short summary of a rather lengthy (13 pages) review of the iPhone 5s posted on Anandtech by Anand Lal Shimpi. In his article, iPhone 5s slaughters its quad-core rivals in performance tests despite having "just" a dual-core CPU and 1 GB of RAM. Zach notes that this is the "most thorough [technical] review" to date. Shimpi's article quickly grew to 40 pages with the addition of user comments.
Falling into the speeds and feeds trap when discussing mobile devices can be tricky, especially when the specs that smartphone vendors share most frequently only paint a small piece of the picture. It's easy to look at things like CPU cores and clockspeeds and make assumptions, but dozens of factors contribute to a smartphone's performance. --Zach Epstein
The iPhone 5s is the first Apple device (and consumer device in the world) to ship with a PowerVR Series 6 GPU. The G6430 inside the A7 is a 4-cluster configuration, with each cluster featuring a 16-wide array of SIMD pipelines. -- Anand Lal Shimpi
The only negative bit I read that I think is worth noting for a general user is how much wear his Apple case showed after only one week of use. That photo appears on page one of Anand Lal Shimpi's review. Many companies make cases, so you might be best off not purchasing the Apple leather iPhone 5s case.
The iPhone 5c Review (AnandTech)
iPhone 5s review (Engadget)