Screen, ports, keyboard and trackpad:
The screen is an absolutely gorgeous 2560x1440p IPS touch panel at 12.5"(yes, the same size as the Macbook) that I found myself gravitating towards over every other display I have. Seriously, it looks great. As for the input mechanisms, I found the keyboard surprisingly good for such a thin laptop, actually preferring it slightly to my Macbook Air keyboard, and the trackpad is not bad but not great. Meanwhile, Lenovo's beating Apple and HP's Spectre by including not only a USB 3.1 Type C connector but a standard Type A USB 3.0 port and a separate power port that can be used as a USB 2.0 connection. Sadly the charger is not Type C, so if you have a phone with one of those new connectors you can't use the same charger.
As I opened with, the Yoga 900S is a very, very small laptop, and is as you might expect therefore not extremely powerful. However, Intel's done a great job updating their Core M processor line from last year, when it was in its first generation. The new Core M's are based on their 6th generation Skylake architecture, and the top of the line Core M7 in the Yoga is just about as fast as my Haswell Core i7 ultra low voltage chip in benchmarks.
All of that nerdery aside, unlike a Lenovo I reviewed last year with the old Core M, and the 2015 Apple Macbook, the Yoga 900S keeps up with normal usage just fine. I never felt any major speed penalties compared to even my desktop when using lots of Chrome and Edge tabs, Notepad ++ and a few Windows Explorer windows. Similarly, the graphics are much improved - streaming a 4K video from YouTube with a bunch of other background tabs open was absolutely fine. Besides the Core M7, there's 8GB's of DDR4 RAM and a 256GB solid state drive. All of that said, if you want the laptop equivalent of a Corvette, the 900S is not your computer - it's more like a Miata. In particular, gaming is out; Even something like Rocket League was too stuttery to be playable.
There's essentially 3 main direct competitors to the Yoga 900S: The Apple Macbook, ASUS Zenbook 3 and HP Spectre. The Macbook is slightly better looking in my opinion, and has OS X which is a huge advantage for some, but there's some severe usability issues with only having 1 port. Additionally, it costs a full $300 more for the base configuration. The Spectre is even thinner than the Yoga, yet somehow still fits in higher powered Core i processors. But it's pricier at $1169, has a lower resolution screen, and no touch capability whatsoever - I'd take the Yoga's beautiful 1440p screen over a slightly faster CPU. Finally, while the Zenbook 3 looks great, it's not out yet, so I can't really recommend it without some evaluation time. Of course, you could also move up to a standard, less crazily thin ultrabook like the Dell XPS 13, normal Yoga 900(the non-S version) or Macbook Air, but that's a different category of device. That leaves the Yoga 900S as the winner, unless you hugely prefer OS X.