I have Windows 8 on my laptop, and overall I like it. It feels much snappier than 7 did (Not sure if it's simply because it's a fresh install, or lack of Aero?), and boot times are really good.
However, the entire Metro UI feels half baked and like a last minute addition. Opening, and then closing a Metro app does not take you back to where you were, it forces you back to the start screen. You then have to alt-tab back to the window that you were at. I had to Google where the Shut Down option was (Yes, I know that I can just press the power button, but I very rarely shut it completely down. I usually either restart, or put it in sleep or hibernate depending on what I'm planning). For the record, the shut down button is now in the Settings panel for some unknown reason.
As far as the start button, there was NO reason for that to be eliminated. They could have kept the metro UI, and start screen, and still had the start button. Removing that and forcing the start screen is just forcing a poorly designed UI on everyone. The really frustrating part is that in Vista & Win7, they had made a ton of improvements to the start menu. It was really, really useful and intuitive. Much more so than the crap they have now.
There's also quite a bit of half baked system settings screens that use the metro UI. For example, in the Devices & Printers screen, you can add and delete printers, but I haven't found any way to do anything else, like adjust settings, view the print queue, etc. Right clicking does nothing. You can still go to the Win7 style control panel, but that's somewhat hidden, and not nearly as easy to access as it was in Win7 (Unless I'm missing something).
Don't even get me started on Minesweeper and Solitaire now. Both used to be games that you could open in a window, and quickly play while you were waiting for a task to finish. Minesweeper is now an over 130MB additional download, which wants you to log in to xbox live to play, forces full screen, and takes several seconds to load. I know that the games aren't a major part of the OS, but it really epitomizes how much usability has been lost in Windows 8, and how they're trying their hardest to force metro UI on everyone.