Joshua Ho and Ryan Smith pulled together an incredibly detailed analysis of the iPhone 6s and 6s Plus for AnandTech.

As always with AnandTech reviews, look for the table of contents popup menu at the bottom of the first page (above the comments). It lets you jump between the 15 pages that make up this review.

From the conclusions page:

The iPhone 6s in a lot of ways seems like it’s simple enough to review, but it turns out if you dig deep the changes have been significant. Over the course of a review, we’ve found major changes in the SoC, storage solution, camera, touch screen, fingerprint scanner, voice recognition software, cellular architecture, and WiFi chipset.

On the SoC side, it’s pretty safe to say that the A9 SoC is the best SoC in any phone today. We can talk about the TSMC and Samsung controversy, but at the end of the day regardless of which one you end up with the performance is going to be far and away better than anything else we’ve seen thus far.

And:

The move to a 12MP rear camera was something that I personally was at least mildly skeptical of, but after testing the camera for myself I’m firmly convinced that Apple has managed to move to 12MP without noticeable degradation. The camera may not be sharper in most scenes, but the extra pixels enable 4K video recording, and it seems that this generation the improvements to video recording quality are enormous. On both iPhone 6s’, the addition of 4K video recording without random recording limitations, loss of image stabilization, or rapid overheating is surprisingly rare given the number of phones that support 4K video recording. The addition of 1080p120 slow motion video only magnifies just how far ahead Apple is in this segment when compared to Android smartphones.

This is just the tip of the iceberg. A review well worth reading.