It's here, it's clogging up the internet and it's probably put a few people over their monthly bandwidth caps already.
Yes, we mean Lion, the latest version of Apple's desktop and laptop OS. It promises to bring the best of iOS back to the Mac, and there are some interesting new features too. So is it worth the budget price of £20.99?
We'd say yes, but only if your apps will work on it: for example, we found Logic Pro 8 refused to launch once we'd upgraded; if we were musicians rather than writers, that would have been a disaster. As it is, we suspect an update won't be forthcoming, and Logic isn't exactly cheap.
Expect other irregularities too. Lion decided it didn't like our mouse, and decided not to recognise our magic trackpad in a fit of pique.
These are the joys of early adoption: if you're using non-Apple kit or older software or hardware, we'd recommend checking for compatibility before hitting the App Store. Adobe has already published a list of issues and other firms are likely to follow suit.
Mac os x lion

COMPATIBILITY: Make sure your favourite apps are compatible before upgrading. 

Lion's user interface
The new interface is a nice place to be, provided you don't like colour: Apple has leached the colour out of OS X, with the odd blue progress bar lost in a world of endless greys. You'll probably encounter a few minor irritations too, such as the enormous text size in the Finder sidebar and Mail's folder list; you can fix that by going to System Preferences > General > Sidebar Icon Size.
While you're in System Preferences you might want to change another thing - the mouse and trackpad settings, which are set to move content in the direction of finger movement when you scroll - and then use View > Customize to remove any System Preferences icons you don't need.
Mac os x lion

SYSTEM ICONS: Get rid of icons you don't need in System Preferences with the handy Customize option
Windows are now resizable from any edge and scrollbars have become dinky grey lines, and if your app supports Lion's new full screen mode - for now, most don't - you'll see a double-arrow icon at the top right of the application window. Clicking it takes you into full screen mode and Esc takes you out again. Moving the mouse or trackpad to the top of the screen when you're in full screen mode displays the menu bar.
Mac os x lion
FULL SCREEN: iPhoto is great in full-screen mode on a 27-inch iMac. Mail, not so much

Mission Control and Launchpad
The new Launchpad screen addresses the issue of finding applications that aren't in the Dock without having to wander around your Mac's hard disk, and uses a distinctly iOS-like interface to display all your application icons and folders. If you're used to a dedicated launcher you'll hate it, but if you aren't - or if you're new to the Mac altogether - then it makes perfect sense.
Mac os x lion
LAUNCHPAD: Launchpad bridges the gap between the Dock and faffing around in Finder. Non-geeks will love it

Mac os x lion
FOLDER SUPPORT: Launchpad supports folders too, so things never get too cluttered on screen
Mission Control, on the other hand, is more for power users. A hot corner, function key or multi-touch swipe displays your open applications, your applications' windows, any full-screen apps, Dashboard widgets (optionally) and Spaces, enabling you to quickly find what you're looking for. The interface is in 3D, too, albeit subtly.

Mac os x lion
MISSION CONTROL: Spaces, Dashboard and Expose together in one happy 3D family

Lion's default apps
The revised Calendar and Address Book apps look like their iPad equivalents, which has already divided opinion. We think they're horrible, but we'll get used to them.
Mac os x lion
IPADALIKE: Address Book gets an iPad-style makeover. It's a rare blast of colour in Lion's mainly grey palette
Mail's iPad-style overhaul is much more successful, with grouped conversations and a distinctly minimalist UI. The folder list is hidden by default but you can unhide it with a click if you miss it.
Mac os x lion
NEW MAIL: The redesigned Mail app is more useful than before, and once again there's a strong iOS influence to its UI
Mac os x lion
NOT KEEN: The new Calendar looks like the iPad one. We don't like that one's interface either
Safari gets some under-the-hood enhancements including a newer version of the WebKit rendering engine and improved security against browser exploits, and the downloads window has been replaced by an iOS-style toolbar icon and pop-up.

Mac os x lion
DOWNLOADS: Safari's Downloads window is no more. Instead, there's a toolbar icon with a pop-up, shown here

The big new feature, though, is Instapaper - er, Reading List. Whenever you see something interesting online, just click the Read Later - er, Reading List's Add Page button - and then load it up in Instapaper - er, Safari - when you've got time to read it. Come iOS 5 you'll be able to sync your Reading List with your iOS devices, just like you can with that program whose name starts with I and ends with "nstapaper".
Reading list
Then there's DropBox - er, AirDrop. Like DropBox it enables you to drop files to other computers, although for now it's a Lion-only feature (DropBox is multi-platform). It benefits from a typically pretty Apple UI, but if you're already using DropBox there's nothing particularly exciting here.

Versions and AutoSave
Lion introduces a new way of saving data, although your apps will need to be updated to support it: files are saved automatically with no input from you, and once you've given your file a name you'll see Save a Version in the File menu instead of Save. Clicking on a file title in its menu bar then gives you the option to lock a file so it's no longer autosaved; to revert to the last saved version; or to browse all the different versions, which launches a Time Machine interface for that specific document.
VERSIONING: If your apps have been updated to support it, Lion will automatically save and track files for you
It works very well, but it'll be a while until it filters through to all your apps - and probably even longer before you trust it, because it feels really weird not hitting Command-S all the time. The autosave also means that if you accidentally quit a program, you can just reopen it and pick up from where you left off - and when you log out, your Mac will ask if you want to restore your open apps when you log back in.

This is a hands-on rather than a full review, but it's clear that there's enough in Lion to make it a must-purchase for most Mac users - although it might be worth waiting a while, because most apps haven't been updated to take advantage of new features such as automatic saving and file versioning.
Watch out for incompatibilities, too: as we discovered today, just because something works on Snow Leopard doesn't mean it'll work on Lion.