There's a little bit here if you're an intense user of some applications, but the big story is a possible vulnerability that would let a hacker steal your Keychain contents.
Update 4 p.m. Sept. 25: A previous version of this story advised readers not to update due to a Keychain security vulnerability reported by ZDNet earlier today. Turns out this vulnerability is in a lot of previous versions of the operating system too, so we've changed our advice. Whee!
Of course, now it makes the update a little easier to sell, ostensibly to get cumulative security fixes, though you'll still get fixes for Sierra and some other older versions.
Otherwise, under-the-hood updates can be tough for software developers to push, especially for operating systems, unless they noticeably increase performance or stability. They simply can't answer the question of what's in it for me very persuasively. That's why you'll hear a lot about the few new features such as Safari's autoplay blocking and the ability to edit Live Photos in Photos, and really fast file copying on SSDs.
All of this makes MacOS High Sierra a pretty tough sell for Apple, since there's very little here that's going to obviously change your experience for months, despite Apple's best efforts to convince us otherwise. There are just a handful of quality-of-experience enhancements if you're a big user of those applications.
Performance is virtually identical, at least on an up-to-date MacBook Pro (13-inch). File moves are instantaneous under both High Sierra and Sierra, but copies of large files (like a 4.3GB ISO image) are instantaneous on AFS. That's really important if you work with video and other massive-file generating tools.
Battery life seems to be slightly better -- we got about 30 minutes more. But it also turns out that High Sierra breaks one of our benchmarks. Oopsie.
Otherwise, it's more that there's little reason not to update, except in a few cases. Though my standard recommendation is to wait at least a month before updating and let the early birds find all the problems.
High Sierra has the same requirements as Sierra, so if you're running that now the answer's yes. If you never updated to Sierra, check out Apple's compatibility list.
There's a lot going on under the hood to lay the groundwork for future enhancements, though much of their benefit doesn't appear at the moment. When it comes down to it, many of them Apple really couldn't put off.
In order to be able to work with iOS 11's new file encodings -- the HEIF (photos) and HEVC/H.265 (video) which allow for better compression to save space on your iPhone -- Apple had to update MacOS to understand them.
In addition to the aforementioned reasons AFS is necessary, it also theoretically improves performance and security. That's always a nice perk. But despite having over a year to work out the kinks with AFS, Apple rolled out High Sierra with a big caveat: AFS will only work with SSDs for now. You shouldn't use it for HDD+SSD Fusion drives and regular HDDs (hard disks). Don't even think about it. When High Sierra went final, beta testers who had converted non-SSDs to AFS were greeted with a mind-bending list of instructions for banishing AFS from their systems.
HDD-supporting AFS is definitely coming, but we don't know when. But it means the systems which need the performance boost the most don't get it yet. It also means you can't use it on most drives used for backup, so no performance boost there.
On the other hand, if you do have a system with an SSD, AFS delivers noticeably better speed, at least for same-disk file copies for GB-size files, and security that's probably worth the update now rather than later.
Architectural changes like a new file system or changes to permissions -- yup, there are changes to SKEL (Secure Kernel Extension Loading) aka Gatekeeper -- may make it difficult or impossible to install some applications in the beginning. Luckily, this workaround seems to still work. So make sure your most prized third-party applications will install before you commit. (Your currently installed ones should remain installed.) For instance, I use VMWare Fusion to run Windows and that won't be fully compatible until October.
As I mentioned earlier, if you plan to take advantage of the extra space savings offered by the new photo and video file formats, you'll have to update to MacOS to be able to view or edit them on your Mac. You don't have to, though; if you prefer to keep it compatible, just go into Settings/Camera/Formats on your iPhone and change it from "High Efficiency" to "Most Compatible."
If you're a big Photos user, Apple has certainly improved the organization and editing interfaces to make using the software more streamlined, and added the same Loop, Bounce and Long Exposure effects for Live Photos that you've got on iOS 11. (Unfortunately, on the bigger-than-phone-size screen of a computer, it's easier to see how the effects degrade the quality.)
And now Photos has an extensions interface where other companies can serve up projects for creating books, cards, calendars and so on. All stuff you could do before, but now from within Photos. And it will happily tell you that the book you just laid out will cost $120.
The latest version of Safari has some really nice features, implemented in a way I wish other browsers would -- you can set default zoom levels on a per-site basis and quickly get to those per-site settings right from the main menu, for example, and the Intelligent Tracking Prevention (which expires third-party cookies used to track you across the web after 24 hours) is an easy way to take a basic step toward more privacy.
And of course there's the ability to block autoplay videos as long as they make noise. Apple claims it's also faster, and it might be when measured in milliseconds, but in practice I really don't notice much of a difference bouncing back between that and Chrome.
But those don't require an OS update. On the other hand, some capabilities of Safari 11 do require High Sierra, however, most notably accelerated streaming HEVC video playback. But there isn't a lot of that content available yet to stream.
If you don't already use one of the myriad services available for collaborative editing -- Google Drive, Dropbox, Box, Microsoft OneDrive and so on -- Apple now offers basic file sharing with real-time updates. It also enables the "universal clipboard," which is essentially the same thing but across nearby devices you've got logged into the same account (sort of like Airdrop), but it's hard to tell what's new. And it makes it easier to manage your family plan.
But the rest could have easily been slipstreamed into Sierra without fanfare. Do you use the Touch Bar? Apple has made some "enhancements" to its operation. I put "enhancements" in quotation marks because some of them don't really feel like it. For instance, you can now flick the brightness and volume controls instead of sliding them. But flicking properly doesn't feel a lot faster or easier than just pressing and sliding. The expanded color picker options look pretty, but to use them you have to constantly look away from the screen.
Top Hits in Mail search results? Meh. Split screen message editing in full screen? Sure. A more compact message store? Hell yeah, at least for the few, the proud, the Apple Mail users. FaceTime Live Photos (to capture something on the other end of the call)? More of an iOS perk. The ability to pin Notes and use tables? Big news for Notes users.