Snow Leopard is at the same time quite appropriately, and entirely misleadingly named. Appropriately, because it is, as the moniker betrays, more an upgrade to the previous incarnation of OS X than a radical overhaul. Misleading because while a living, breathing snow leopard is superficially distinct, but fundamentally little different to a leopard, OS X 10.6 is pretty much the exact opposite; little has changed on the surface, but underneath we're dealing with an entirely new creature.
It's worth considering, though, that with its £25 upgrade price, it's unlikely that many Leopard users are going to be in much debate as to whether to go for this new, fluffier version. That amount is certainly well within my impulse-buy threshold, and I surely can't be alone. If you're the frugal type, however, and are sitting on the fence then you'll probably want somereassurance first.
From the word go Apple shows it hasn't been resting on it's Leopard-scented laurels. The installation is "up to 50 per cent" faster than Leopard this time around, says Apple, and I certainly couldn't call it slow. Vista and Windows 7 may also offer upgrade installs from their previous versions, but Apple outright encourages it.
One neat trick of the installer is that if for some reason the upgrade process is interrupted - say, your laptop battery dying - it will simply carry on where it left off with no data loss. It's one of those gimmicks most users will never notice, but which suddenly seems a lot less pointless when you find yourself needing it.
Impressively, a Snow Leopard install will take up less space than the Leopard it has upgraded from - some 6GB on average. Some of this space saving is as a result of compressing OS files, part of it comes from removing Rosetta - Apple's PowerPC compatibility layer - from the default install and part of it comes from not installing printer drivers by default, both they and Rosetta will now be pulled from the 'net on demand.
Any programs installed in Leopard and known to be incompatible with Snow Leopard are put in a separate folder during the upgrade. It's then up to you to either find an alternative that will work, or hope there's an updated version.
The entire core application base of OS X 10.6 has been rebuilt in 64-bit. Well, almost; Grapher, DVD Player, Front Row and iTunes are still 32-bit although only with the later will anyone ever notice, I'd warrant - iTunes is still horribly slow at times.
With Snow Leopard Apple has cut support for any new 32-bit APIs, which should give a clear signal to developers that the time to switch to 64-bit is now. The transition should be made easier, as OS X can run 64-bit applications even when booted into the 32-bit kernel, which it does by default - unlike Windows which can't run 64-bit programs in its 32-bit incarnation.
There is one limit to the 64-bit/32-bit coexistence, in that OS X doesn't cater for 64-bit applications loading 32-bit plugins. This gives rise to one of Safari 4's neatest features. While Safari itself loads as a 64-bit application, plugins in pages are loaded in their own separate processes. A fringe benefit of this is that should a plugin crash (I'm looking at you Flash) it won't take the browser with it - Safari simply reloads the plugin and refreshes the page. It's not quite up there with Chrome's tab-independence, but in the real world both methods will likely prove as effective at preventing browser crashes.