Two weeks ago I bought a copy of Mountain Lion. It refused to install on my Macbook Pro because it conflicted with Lion. My computer got caught into a conflict loop and refused to start. I ran it a couple of times but it kept timing out, then it caused bad sectors in my Hard Drive.
I took it to the Apple Store, the "Genius'" fixed my Hard drive, but that was it. I took it home and formatted the Hard Drive, still the same issue. Found a copy of Snow Leopard and that isn't even compatible with my Macbook. WTF??????
All of this wouldn't be so frustrating if they actually sold physical copies of their ***** OPERATING SYSTEM! HATE, HATE, HATE, HATE, Apple and their Digital Only BS. How am I supposed to download your software when it ONLY runs on YOUR computers and it was YOUR software that f'ed my machine in the first place?
My only option now is to go back to the stupid Genius Bar - nearest Apple Stores are an HOUR from my house in either direction! Drop it off and hope they'll install the stupid operating system for free. All of this because Apple is run by a bunch of *******s who think they know what's best instead of giving customers a choice. I'm NEVER buying another Apple product, even though technically I need it for work.
So let me get this straight:
Mountain Lion has already sold 3+ millions of digital copies, and you advise anyone to stay away from it because it allegedly destroyed your computer? I have been doing incremental updates from 10.5 Leopard. I have never ever needed to install OS X again. And I have been using the computer for all sorts of stuff (gaming, video editing, I also have Windows installed...).
From a programmer's perspective, there must be something wrong before you began installing mountain lion. The problem lied dormant until the Mountain Lion's install touched some files it needs to upgrade and the problem appeared.
It's not uncommon for existing problems to appear during installation requiring a complete reinstall because the user payed absolutely NO ATTENTION to the use of the system, as it's not uncommon for outstanding problems to be fixed with an upgrade, even if the upgrade was not intended to fix those (you know, old corrupted files get replaced by new and working ones).
But in all my years having a Mac (12+) and doing some extreme things with them, I have never ever encountered a problem that would keep me from booting and formatting my disk to install a new OS from scratch.
It can also be a hardware problem, have you ever thought of that? HDDs behave like the way you described a little before they "die", sometimes they behave properly afterwards and then they shutdown and don't start again. I would replace the HDD of this machine immediately.
That said, you should always keep a hard copy of the latest digital copies of the OSs. Burning a copy of Lion and Mountain Lion in an external DVD is extremely simple, and legal. I strongly recommend it.
PS. BTW, regarding physical copies, I, too believe that physical copies SHOULD be provided by apple at least for a small fee. Or at least make the digital installer to copy itself to an external dvd with the press of a button, so that you can keep a physical copy for any situation. I always keep physical copies of my OSes in an external DVD and they always come in handy in difficult situations, especially if you use Linux and Windows where sometimes the "Restore" partition gets deleted (With linux). It's nice to not rely on your computer solely for a restore.