The ZUK team partnered with Cyanogen for the software on the international Z1. It's based on CyanogenMod, which itself is fairly close to stock Android. It's not CyanogenMod though, instead it's Cyanogen OS.
The difference is mainly that Cyanogen and ZUK added a couple of proprietary apps to the software package. And of course, Cyanogen OS is Google-certified, meaning you get Google Play Services out of the box.
What Cyanogen has touched has been largely to add extra customization options where the stock software is limited. So it's not exactly Nexus, but it comes with the full Google suite.
The ZUK Z1 units sold in China will run ZUI instead, which changes the interface slightly so it's closer to skins popular in the country (e.g. no app drawer like in MIUI).
Even if you've been living in a skinned-Android world until now, many things will feel familiar as makers have been course-correcting their skins to be closer to the stock vanilla look.
The lockscreen shows the time and the notifications, a swipe from the top also brings in the notification area. A couple of shortcuts are available in the bottom corners - dialer on the left and camera on the right. You can change those with other apps if you wish.
The unlock options include a fingerprint reader. Positioned at the Home key, it unlocks the phone quickly just by placing your finger on top of it. You can quickly get used to press the Home key to waking the phone and leaving your finger in place for the fingerprint reader to unlock it. It all takes under a second.
Double-tap to wake is also supported and you can enable the proximity sensor to prevent the phone from unlocking accidentally while it's in your pocket.
The ZUK Z1 uses the Trebuchet launcher, which has been skinned to look like the Google Now launcher. There's also the ZUK theme, but you can install many more from the Cyanogen Theme Store if you don't like the defaults.
Note that some themes even include custom looks for apps, not just the launcher itself.
The launcher itself features a permanent Google search field on top and a shortcut dock at the bottom. The app drawer is an alphabetized vertical list with an index at the bottom. This isn't as compact as a grid view and there's no option to change it.
The notification area shows the notifications after the first pull and the quick toggles after the second. Here the Wi-Fi and Bluetooth toggles have been given priority, they are bigger than the rest, but you can easily re-arrange them to fit your needs.
There's a brightness slider in here too, though we miss one thing - the Auto brightness toggle. You have to go into the settings for that.
The Cyanogen-made additions include a small weather widget and Quick pulldown, basically giving you one-swipe access to the toggles from the left or right corner of the screen.
Also here is the User switch. By default you get Owner and Guest accounts and you can easily add more of each. Guest accounts can be prevented from making calls, but not from accessing certain apps. Also note that you can switch users from the lockscreen and each account has different security settings.
So even if you have a fingerprint lock set up, it's easy to jump into the Guest account, which does not need to have a fingerprint assigned to it. Accounts do not share data though, so you can't view photos taken with the Owner account from the Guest account, for example. Still, you might want to set up a more secure lock for the Guest account too. There's no way to automatically switch between users based on their fingerprints though.
Getting back on track, the App switcher shows a 3D rolodex of the running apps and in Google style even keeps the search field. This makes it easy to access even from inside apps - tap the switcher key and type in your query (or say it). Holding down the Home key is faster though.
One of the Cyanogen customizations is that you get extensive control over what each button does. You can add different actions for short, long and double pressing the Home key and short/long pressing the App switcher key. You can customize the Power menu too.
Another option is to enable an on-screen bar with these buttons (it's customizable too). Normally we wouldn't use them when there are hardware buttons, but we like having piece of mind knowing that even if those break some time down the road, it's easy to switch to their virtual counterparts).
Chromecast users will be pleased to hear that there's a quick shortcut for screen mirroring in the notification area.
While digging into the screen settings, you can also change the screen DPI. A smaller DPI fools the software into thinking the screen is bigger than it actually is, fitting a lot more content on the 5.5" screen. And if your eyesight isn't that great, you can move to a bigger DPI, making all apps look bigger.