Microsoft released the Windows 8 preview on Feb 29, 2012 and I installed it the next day. Here are my reactions. I used the Windows 8 Preview in VirtualBox.
No start menu! Once you get off the tiled initial screen (the "Metro" screen), there's no obvious way to get back to it. I later discovered that you can tap the "Start" key on Microsoft keyboards. I use KDE, so I don't ever use that key. The tiled screen covers the entire desktop, so it's significantly more clumsy to use than the Start menu.
After you get off the tiled screen, everything looks like Windows 7 without a Start menu. Doing even the simplest thing is clumsy. I need to start the control panel and adjust the display settings for VirtualBox, and had to start the Task Manager to use File/Run... because I couldn't find the control panel anywhere. (If you ever find a program, you can still pin it to the taskbar, but space there is limited.)
On the tiled screens, the little tiles update themselves. This reminds me of Active Desktop. The problem with Active Desktop was that your applications covered it. The tiled screen is invisible while you are working. So neither one really gives you much benefit.
The applications on the tiled desktop look like they're designed to drive traffic to MSN and Bing. Some of them want you to sign up for a Microsoft account before they're usable. (Really? To see the weather? KDE doesn't require anything like that.) I think the motive behind Windows 8 is not a better user experience, but an attempt to leverage Microsoft's desktop presence into traffic to their web sites.
The only way Microsoft can salvage Windows 8 as a desktop operating system would be to put the Start menu back, and have some way to permanently bypass the tiled screen. This would, of course, be just Windows 7.
The benefits of the tiled desktop are not obvious. Microsoft is hurt by the fact that they're releasing a tablet operating system preview as a desktop operating system. The only thing people are going to want to do is disable the annoying new stuff. Further, people using the preview are probably going to create virtual machines which need their settings tweaked in the control panel, making the clumsiness of Win8 more apparent.
Taken on its own merits, the tiled interface is clumsy, but is similar in many respects to the application launchers in iOS and Android. The biggest difference is that the tiles update themselves on the fly, where the other two operating systems don't.
I can actually see the tiled interface working on small-screen mobile devices, in the sense that it is not any more clumsy than iOS or Android. But Win8 has no significant improvements over existing tablet software, either.
Where Microsoft is making a fatal error is forcing the tablet paradigm onto the desktop. Win8 is not going to be accepted in corporate environments until there is some way to go directly to the Win7 desktop, and the Start menu comes back. The worst mistake any company could ever make is giving customers what they do not want, and Microsoft and Win8 is going to join Zondervan and the TNIV as a business case of what not to do.
Corporate roll-out of Win7 is just beginning, so Win8 is the version large corporate clients will skip entirely. Microsoft has a window of about five years to release Win8, learn from their mistakes, and fix them in Windows 9, which is probably coming 2015-ish, depending on how bad Win8 goes down in flames, so corporations will probably not begin another Windows migration until around 2018 or so after Windows 9 Service Pack 1 appears.
Will Win8 be successful on tablets? For Microsoft, I think it will be, in the sense that hardware manufacturers will license it from MS, which means MS has essentially zero risk if the hardware doesn't sell. The only compelling business case for a Win8 tablet is if it runs corporate software that won't run on an Android tablet. (Think MS Office.) I sure don't see any hardware maker competing with the Kindle Fire (which Amazon.com sells at a loss on each unit sold by all estimates) when the Win8 tablet will be more expensive since the manufacturer has to license Win8.
Win8 may actually be software Microsoft can't give away, if desktop users reject it and no hardware makers want to compete with low-end Android tablets. The fate of most niche tablet operating systems (like WebOS) has not been good.