I've been a Windows guy for as long as I've used computers. I build my own rigs and use them for high-performance video editing, design, and animation. I have also used Mac OSX at my "real job" for close to 6 years now, so I am extremely familiar with both sides.
Windows 8 is not iterative, it's a bold step in a new direction. It packs improvements to Windows 7 under the brand new Metro UI and environment. I figured I'd spend most of my time in the desktop environment since I'm a power user who focuses on the Adobe design suite and other desktop programs, but I find myself using the start screen far more than I imagined.
I was initially frustrated with the lack of a start button & menu in the desktop space, but once I realized I could just press the "Windows" key like I normally would, and then start typing what I needed, I found it much more helpful than the typical start menu. For example, a search for "Power Options" yields categories of results like "Settings" or even different apps I could search within. In this case, I could click on "Store" and it would do a search within the store. These changes actually make it a welcome improvement to what I thought was the biggest drawback.
The new start menu does take a bit to get used to, and the limitations of the apps within it can be frustrating, but I rarely find myself deciding I have to switch to the desktop side because I can't do what I need to.
I also appreciate the changes to more functional elements like the transfer window and task manager. It makes everything work much nicer.
Pros -New UI with functional design in mind -Noticeable performance improvements -An integrated Microsoft experience across all platforms (e.g. Xbox & Skydrive) -A relatively inexpensive upgrade
Cons -Learning curve -Not an must have, since 7 was good -RT & Pro will undoubtedly confuse most people
Overall, I'm very happy I upgraded, but I can't necessarily say that it's a no brainer. There are good things and bad things, and I really believe the updates Microsoft is working on will make significant improvements. But until then, you're probably just as fine sticking with 7 unless you'll really make use of the new features.
Verdict: if you're on Windows 7, check out the features and see if it's compelling for you. If you're on anything before 7, upgrade to 8 now!