The Liquid X2 boots Android 5.1 Lollipop with an Acer layer on top. The company's skin leaves most stock interface elements unaltered, but does introduce changes in some areas, and those aren't necessarily all for the better.
Anyway, the lockscreen is very much standard-issue, with a clock, notifications and shortcuts to dialer and camera, which work with any side swipe, not just from the corners. Past that are the usual homescreens with folder support and an app drawer - unlike many makers that put all apps on the homescreens Acer sticks with the two-tiered approach. There's no search or sort functionality in the app drawer, though.
We have a minor niggle with the folder implementation, which hinders their functionality. They are represented as a deck of icons, and you can only clearly see the top one, and barely distinguish the next two, and that's it. Why not have 6 or 9 tiny icons, so you forget what's where, you can tell by color and shape?
The leftmost pane next to your homescreens is sort of an information hub which can pull news for you, remind you of incoming events and aggregate social network messages. You can even set it as the default homescreen.
The task switcher is the regular Lollipop one, with a button to kill all apps.
The notification shade is one of those interface elements Acer could have done better. Pulling it down, you get 7 toggles and shortcuts on top, followed by notifications. You can't pull a second time to expand the toggles, you need to press the "More" button, which also takes up a space for a toggle.
The brightness slider is accessed from there as well, and there's no Auto toggle. We know there isn't one in stock Android either, but if you're going to change something, why not make it better.
On a positive note, Acer has added multi-window functionality and float apps. The former works a lot like Samsung's implementation, but you can initiate it in a different way.
When you're in an app that supports multi-window, an icon appears to the left of the back button in the navigation bar. You tap that and the app shrinks to half the screen size with the other half offering you a choice of recently used apps and a complete list of all apps that support it. You can resize the windows, and swap them, and it also works in landscape. You can't have two instances of the same app, though.
On top of that (literally) you can have what Acer calls Float apps, and Sony calls Small apps. Well, those are small floating instances of the regular apps, which display above all other content on the screen, and include a simple calculator, calendar, browser and camera, among other things.
Remember that flip cover we mentioned in the hardware section? When the phone is in standby, the screen is off, but wake it up and it displays a clock and battery lever indicator. Double tap and you launch the camera, though the tiny strip is perhaps not the best means for accurate framing.
There's more to it, though. From the default state you can slide up or down and it gives you some neat shortcuts. You can enter airplane mode, fire the flashlight or mute the sound. Additionally, when you're on the weather panel, a press-and-hold action gets you a minute-by-minute weather forecast until the end of the day.