Installation and general performance
As in years past, you'll install the Mojave through the Mac App Store. The whole process didn't take much more than an hour on a 2017 15-inch MacBook Pro, but your mileage will vary depending on how fast you can download the installation file and how fast your computer can chug through the update. The oldest computers that can run Mojave are 2012-vintage iMacs, Mac Minis, MacBook Pros and MacBook Airs; if you're running one of those, the installation will probably take a good bit longer.
Immediately after the update, my machine's overall responsiveness seemed slightly slower, but that may have just been the computer adjusting things post-installation. The next day, things seemed just as fast and fluid as they were before the upgrade. Unfortunately, I haven't tried the update on an older machine yet, but our editor-in-chief, Dana, has been running the beta on an already slow iMac we have lying around (one that uses a traditional hard drive), and she has some regrets. We can't say exactly how things will work on your computer, but it's probably safe to say that if you upgraded to High Sierra last year without incident, you'll be fine this time too -- especially if you're using a Mac with solid-state storage.
Embracing the dark side
Mojave's new Dark Mode is the biggest UI change for macOS since 2014's Yosemite update, which ushered in fresh icons, translucent effects and new fonts from iOS 7. If you don't enable Dark Mode, Mojave looks essentially unchanged from High Sierra. But turning on Dark Mode (and pairing it with a dark desktop picture) does make the OS feel different. Apps like Notes put text on a black background, while Calendar takes the colors of your various events and makes them seem a bit more neon against the dark backing.
My impression of Dark Mode is generally positive. It's attractive, and a nice way to change things up in a UI that has otherwise remained unchanged for several years. Also, seeing some of my daily apps like Photos, Messages and iTunes get a re-skin is just fun (if you're a particular kind of nerd, I guess). I hope that developers take the time to adopt the new UI, though, because right now the overall experience feels a bit disjointed. It can feel jarring jumping between Apple's updated apps and ones that aren't, like Chrome.
More problematic is the fact that so much of what we do on computers these days is so unavoidably... bright. As I write this, I'm using Pages in Dark Mode, but the white background of the text entry window is simply screaming against the dark window elements in the app and nearly every other one I have open. The same happens when I browse the web: so many webpages feature text and images on a white background, which makes the transition to darker apps feel abrupt.
Mojave has a few other UI tweaks worth noting. A new dynamic desktop feature adjusts the lighting and color of two wallpapers throughout the day. The basic Mojave Desert scene essentially tracks where the sun would be throughout the day and night, while "Solar Gradients" does a good job of emulating a clear sky as the sun rises and sets. Meanwhile, the familiar Dock now has a section that shows recently used apps, just like the Dock on the iPad. Being able to see recent apps is definitely useful, while the dynamic desktop is just fun to check out every hour or so. Unfortunately, right now there are only two dynamic options. Apple added dynamic wallpapers to iOS 7 back in 2013, but seven options there haven't been added to since then. I hope Apple doesn't similarly abandon dynamic desktop on the Mac and adds more options for it in the future.
I've mostly talked about visual tweaks, but there are several new features worth trying in Mojave. The Finder, perhaps one of the oldest macOS interfaces, has received a significant upgrade. The new "Gallery view" shows a filmstrip of a folder's files at the bottom, with a large preview taking up most of the window. It's a much more functional version of the aging Cover Flow interface it replaces. There's also a new right-hand pane showing full metadata for the selected file, including EXIF info for photos. Much like Dark Mode, this new Finder layout seems particularly well-suited to those working with images or video, but it shows readable previews of PDFs and text documents as well.
The Finder also lets you make small changes to files directly in Gallery view. If you have an image selected, you'll see options to rotate, mark up or convert to a PDF; if you're looking at video, you can trim the selection without even opening it. Third-party apps can include extensions that can be added to this edit area too. Finally, if you're not browsing files in Gallery mode, you can just hit the space bar to bring up Quick Look and do the same edits there.