Summary: This is the Windows XP today. It could have been called Windows XP2 and it would have worked. I've been using Windows 7 since the first Beta versions and I love it. I recently had the opportunity to move to Windows 8 at a very low price, and decided to stick with Windows 7. While Windows Vista always felt like an incomplete OS to me, Windows 7 is polished and finished. While Windows 7 can sometimes ask for more RAM than you think it should, performances are great.
Pros: Very stable, Good looking, Have good backward compatibility
Pros: To all those confused with activation, please see my "other thoughts."
Cons: Its expensive in my opinion. They make too many versions and the average user gets in trouble not knowing what they are getting at the store (which is always "home" versions) which then you have to upgrade to get alot of domain / networking functionality.
Summary: Read reviews on Windows 8 and it looks like they screwed up in a number of areas. I'm building a computer which will be primarily used for Adobe CS6 and Lightroom 4 and I know Windows 7 works fine. Also, Microsoft has been following a simple pattern of creating a good OS followed by a bad one. The most recent looks like this: XP (Good), Vista (Sucks), 7 (Good), 8 (So far, looks pretty bad). I'll wait for Windows 9 before I'll consider upgrading to a new OS.
Pros: 'Traditional' Windows design. Easy to understand and navigate.