I am gonna share my experience about upgrading Windows 7 Ultimate to Windows 8 Consumer Preview.
First, I downloaded the installer of course. It's only a small 5MB exe file.
The following are the series of screenshots I captured while installing the Win8.
Before installing itself, Win8 installer will make a compatibility report on what applications are supported and not supported in Win8.

The 5 items mentioned in report are the following including Windows Security Essentials. According to this post the Security Essential is no longer needed in Win8 because the new Windows Defender built for Win8 works like MSE.

Instead of uninstalling those mentioned items, I continue with the Win8 installation wizard. At this point you would want to cancel the wizard and uninstall those incompatible items first before running the wizard again. For MSE, later the wizard will ask you again to uninstall, by the way. For SQL 2008 incompatible features, I dont' have the list of those features that will not work.
Next window.

Next. It's not really 20 minutes but rather about an hour.

Next window. After downloading necessary files.

Next window. License Terms. Accept it.
Next window. Keep everything.

Next window. Here's asking you to uninstall Security Essentials again. At this point, I decided to uninstall it.

After removing Security Essentials, the installer will ask you to restart.

After restarting the machine, the installer will ask whether to continue or start from beginning. Choose 'Continue... '.

Finally the Install button. It took me about an hour till installation completed.

After installation, it will ask for regional settings (I wonder because I opted to keep my settings). It will also ask for your Live account to sync some of your applications with Windows Live (you can skip it, BTW). And some other things.
And finally the Windows 8 start screen.

First thing to notice is that the start menu on the bottom left corner is gone. It was replaced by the above metro UI design.
That's all for now. Let's explore this new Windows.