I've been using Windows 8 as my main OS for the last few days. I was skeptical at first but now I'm really warming up to it.
My 2 biggest gripes:
1. No easy way to shut down! You either have to it from the desktop settings menu, or by logging off, unlocking the computer, and selecting the shutdown option.
2. Apps don't always seem to suspend properly. Some games keep running in the background even if you exit out of them. You have to open another app before they suspend.
One minor gripe/observation:
You have to go to the Start screen to launch a desktop app? It seems a bit jarring to have to leave the desktop entirely in order to open a desktop app.
Overall, though, I'm excited about Windows 8. I love the metro interface with live tiles. I definitely love the way Metro apps can suspend/resume so amazingly fast. It's great for performance/battery life. I love the idea that all Metro apps will be able to run on x86 and ARM.
It looks like Windows 8 will have some Touchpad-like features as well. From a leaked image, it looks like you will be able to answer calls/view missed calls, manage text messages, etc. from your Windows 8 device.
Oh yeah and one other thing: I took a picture with my webcam to use as my Windows 8 user picture. Then later I noticed the "Me" tile on my Windows Phone. . . it was updated with that picture! Crazy!