I signed up for the Apple Beta Software Program and got an early taste of the latest iOS. It supports the iPhone 5s, iPad mini 2 and later, and the iPod touch (6th generation).
The device used for this writeup is an iPhone X. Some screens may look different on other devices, so please keep that in mind.
What Changes in iOS 12?
- Improved overall performance
- Faster Camera app launch speed
- New "Measure" app
- New "Screen Time" feature
- Partial UI design tweaks
…and more.
The New "Screen Time" Feature
Screen Time is a feature that lets you block apps from being used during certain times of day. It feels close to parental controls, and you can restrict the use of most apps with a dedicated passcode.

App time limits can be configured in fine detail, so you can stop yourself from using your smartphone late at night, for example.

Once the time you set rolls around, the app simply won't launch.

Measure Distances with the Measure App
A new app that suddenly showed up on the home screen, this one uses AR to measure distances. You can measure size starting from a point you tap on the screen.

Pretty handy when you don't have a tape measure nearby!?
Something Everyone Was Waiting For? Closing Apps Gets Easier
On the current iOS you have to long-press an app and then tap a button to close it, but iOS 12 looks like it's bringing back the old swipe-to-close gesture.

Overall Impressions
The features above are only a small slice of what's new — ARKit has been upgraded, Memoji have been added, and there are plenty of other changes. Combined with the performance improvements, this is shaping up to be an update worth recommending.
That said, this is still a beta, so there may be bugs and some apps may stop working. Be careful before installing it on your main device. Since the update is supposed to improve performance, why not try the latest iOS on a spare device you're not actively using?