A lot of what I do the past 5 years is test, experiment and basically dig deep into how our tools of the trade actually do their job. When it comes to computers that means literally taking apart the OS and either reverse engineering it's core setups or worse, turning a good running system into a barely running shadow of it's original self - non-destructive testing for software I suppose you could call it. But all this tweaking also means I have to then restore all my bungled surgery back to it's original condition so my once pristine system regains it's former healthy status.
Before Snow Leopards' release, restoration was an often painful task (and forget about any version of Windows, painful doesn't even come close to being a good descriptive) simply because the restore process really didn't take me back to a 100% restored state but more of a hybrid of good code with bad sitting in an old folder - just in case you end up having some parts of the OS that require needing it's parts manually replaced.
Right away some people are thinking, "Why not simply make a backup prior to all this testing?". But therein lies the point: Many people get themselves into a situation where they've gotten their system in a poorly running state or worse, had corruption caused by user error or a poorly written app that's gone amuck. It happens even in Mac-land. These same people often do *not* have any backup strategies in place so all they have is their single install on their main machine - and they're desperately trying to fix their broken or crippled OS.
Well fear no more: Snow Leopard is now *nearly* 100% self-healing. I say nearly, because nothing replaces having a stable clone or backup OS on safety-standby but now you can fix things quickly without the fuss of before...
If you're running Snow Leopard and get into trouble with OS stability you now only have to do one thing: Insert and boot from the OS disk and re-install the OS. That's it.
Worried you're going to erase your previous installation and all your apps or, like the previous OS's where "Archive & Install" simply put a new copy of the OS and moved the old off to a new folder? Worry no more.
In fact what happens is quite brilliant: Snow Leopard's install routine is now by default (as in hard-coded into the install routine) always in "Analyze and Replace" mode, no more archiving old code (although technically it does move a few things into a safe, hidden folder). What happens is this: The install routine looks at all the current code installed on the machine, analyzes which code is good and which is bad and simply erases old core files and replaces them with original code and puts all core services back to a fresh install state.
But the best part: It does NOT erase any apps, preferences... nothing. All your previous data is kept in place - as long as those user-created core OS files are not corrupted - and not moved off to a "previous OS" folder. (no effect on third-party software prefs at all). That means you get back an entire OS - albeit reverted back to 10.6.0 - but in perfect running order and exactly the way you had it setup. All you need do is run Software Update to bring you current to the OS state and you're back in business. End of story.
I'm often very critical of what Apple doesn't do properly, but their golden child, Mac OSX, has just gotten brilliantly better for us all.