The "System Builder" discs of Windows 8 are named this way because they are a full install and not an upgrade. The assumption is that you are building your own machine and looking to put Windows 8 on a from-scratch install, but this is still a multi-boot friendly OS and if your intention instead is to install a full copy of Windows 8 without upgrading your current system, well, that works too: you can put this on old hardware so long as your machine meets the system requirements. There has been some confusion at the lack of an 'OEM' SKU for Windows, so I suspect that until we hear otherwise, this is our 'OEM' substitute for those of us who like to tinker with hardware.
If you're considering Windows 8, there are benefits many small and large. The patchwork way in which the OS seems to meld old into new can be confusing, and it's clear the UI pays lip service to mice while eagerly awaiting your first touch-screen or touch-pad purchase. But to give credit where credit is due, there are benefits under the surface to go with the drawbacks you see before your eyes. Putting Windows 8 Pro on a new system is a good hedge against longevity, but will require experienced Windows 7 (and below) users to be patient with the newer parts of the UI.
This version of Windows 8 is different from Windows 8 Pro in that it does not allow you to use Remote Desktop or BitLocker hard drive encryption, nor can you join the machine to an Active Directory Domain. There are several native Windows 8 apps included with all versions of Windows 8: News, Stocks, Weather, Pictures, E-mail, Music, Xbox integration (to your XBox account, to view your stats or stream content to/from your PC), and Facebook. There is also support for Windows SkyDrive and a Windows Live account, to the point that your PC's user account can be fully integrated with your Windows Live account if that is what you wish. While Windows Media Center is listed as requiring a separate license, right now Microsoft is offering that license for free on the Windows 8 website (Amazon won't let me put external links here or I would).
The biggest change in Windows 8 is that the Start Menu that we have had since Windows 95 is no longer a menu. It is a full-screen splash of square tiles instead of icons, which vary in width from one to two tiles wide. Some of this makes sense in that some of today's programs are more like the Windows Desktop Gadgets we've seen in Vista and 7: they sit idle, stream information and need to be big enough to be readable. Although this is the biggest visual change, if you've used Windows Media Center, Office 2010, or an XBox 360, you've already been interacting with similar interfaces. This sort of UI has been slowly making its way into Microsoft's products for a while now.
That said, the transition from the familiar Start Menu to the full-screen splash takes some getting used to. It may take some arranging to get the tiles laid out conveniently, but Microsoft makes this easy to do. The rest of the Windows 8 OS spends its time hopping between the old and the new look. The desktop looks exactly like it did in Windows 7 and Vista, except there's no Start Button: you're expected to press the Windows key or move your mouse to the bottom corner of the screen to launch the tile dashboard. The desktop looks exactly as it did in 7, only without the Start Button. All programs can be launched directly off the tile screen, and while Windows 8 apps run one at a time in a full-screen view, older programs run from within the Desktop and can be resized. We have a version of Internet Explorer 10 accessible from the desktop that looks just like IE 9 did in Windows 7, then we have a version of Internet Explorer 10 in the tile screen that looks entirely different. Windows 8 applications don't really "close" any more: when you leave an application, it just disappears from view, like a smartphone OS would do. Some parts of the Control Panel have the tile look and feel and the options cascade left-to-right like the Home screen in Office 2010. Other parts of the Control Panel look exactly like they did in Windows 7 and before.
Windows 8 may appear to be a patchwork of old and new, but there's more going on under the covers. In earlier versions of Windows, the number of programs running in the background could get unwieldy and slow things down. There was a shift to rely more on Services, which are listeners that sit idle waiting to launch programs only when needed. Windows 7 started to get bogged down with a lot of running Services though, and so Microsoft have stripped several services out of Windows 8, which means Windows 8 runs more efficiently than Windows 7. Any machine that can run 7 can run 8 (and I remember installing 7 on some pretty old hardware!).
Where older versions of Windows would have to re-launch a program every time it is opened, in Windows 8 anything not being used by you is set aside and the resources it used handed back to the OS for other work. Programs respond much faster on relaunch due to having been left in this standby mode. The Windows Vista and 7 "Aero effects" that gave your windows a glassy sheen and rounded corners used graphics and CPU to run, so they've been stripped out in favor of simple color schemes, a bit of glassiness at the Desktop, and blocky corners. Interaction with Windows in the Tile UI is easier if you have a touchscreen or you're using a tablet, and if you're a Windows Phone user the territory is already pretty familiar. The downside for me has been that I don't have a touchscreen, and so I spend a lot of time hunting along the edges of the screen for scrollbars and dragging around to be able to interact with everything. My chief complaint with the UI hasn't been with the Tile dashboard so much as it's been that the scroll bars are too skinny and at times I have to hunt for them.
Ultimately, time will tell whether people adjust to Windows 8 or long for the familiar. I've found the IE 10 browser on the Tile side to be less compatible and more cranky with websites, but I expect that to change given time. I like the fact that antivirus and security are rolled into the OS without me having to take care of them or be interrupted with update notifications. The OS runs updates on a schedule that's far less aggressive than previous versions and thus far hasn't forced me into a reboot, and Microsoft has partnered with Adobe to roll their updates into Windows--no more random and frequent Adobe update nags. I've been told that the performance for gaming is supposed to be greater simply by virtue of a cleaner codebase, but I haven't seen anything substantially different apart from faster boot-up and shut-down times. Gamers remember that you're also at the mercy of the game-maker--just as we've been with any OS release. The basic applications like Weather, Photos, Stocks, Mail, and more are very user-friendly, but solely within the context of the new look & feel: expect little familar ground and a lot of new territory.
UPDATE: Amazon customer Robert Haines points out (in the comments) that a program called Classic Shell will remove the Tiles interface and restore the 'classic' Start Menu. There is also a program called Pokki that will let you customize your own UI. A. Humphries also gives us a link in the comments below showing Stardock's product "Start8" and some good feedback on how it works.
UPDATE: Amazon customer AJ points out that although Windows 8 Pro may in the future require you to purchase a license for Windows Media Center separately, right now you can get a serial key for free via the Windows 8 site: just go to the Windows 8 "Add Ons" section. You may request up to 5 serial keys using different e-mail addresses and Microsoft will e-mail them to you within 24-48 hours.
UPDATE, 11/2013: Microsoft has now released Windows 8.1, a minor update to the OS that mostly attempts to make the user experience better. If you purhcase this copy of Windows 8, you will be offered the ability to update it to 8.1 at no additional cost. The upgrade is pushed through the Windows Store that is part of Windows 8. Personally, I've done clean installs of 8.1 from the MSDN media and done an in-place upgrade via the store, on both my desktop and laptop, and have not had good experiences. I don't personally recommend the update at this time for that reason. Peraps with time it will get better.
This review was posted under the System Builder DVD product listing of Windows 8. The three versions of Windows 8 are this "just Windows 8" version (which is a bit like Windows Home in earlier versions), "Windows 8 Pro", and "Windows 8 Enterprise". The generic Windows 8 that replaces "Home" in earlier versions has the basic feature set. The "Pro" flavor adds the BitLocker drive encryption we previously had to buy Windows Ultimate to obtain. It adds the ability to use Remote Desktop (that in my opinion should be in all versions of Windows), and it makes it easier to connect your computer to an Active Directory Domain. Windows 8 Pro is best for the power user or the small business and most people hardware-savvy enough to build their own machines will most likely prefer the added control they gain from using Pro. If I were building a system for a non-savvy family member I might go with this standard version of Windows 8 though: the likelihood of needing drive encryption or remote desktop on a system that would mostly be used for documents and web surfing isn't much, and I certainly don't have family or friends running their own AD Domains. "Enterprise" takes things one step further by being designed for use in medium to large businesses with corporate networks. There is also a "Windows 8 RT", but this version is solely intended for the ARM processors and other hardware built-in to tablet computers and is not intended for a desktop or laptop computer.