I installed the Windows 7 RC on all 3 of my computers when it first released back in May. I have an old HP desktop, a new HP laptop, and a three year old Dell laptop. Before the RC, I was using the Windows 7 beta on the Dell laptop. I have to say, I'm totally blown away by the quality and stability of this OS. I've tried a few versions of Linux, tried OSX, used Vista, and have had XP as my primary OS for many years. Windows 7 is far and away the best out of any of these. In the time I've used 7, I've had not one single crash or major glitch on any of my comptuers.
The Windows 7 core seems to be incredibly stable in all of the testing I've done with it, and Windows 7 has performed even more solidly across all 3 of my computers as updates have been released. My only complaints at the beginning were issues with the Aero effects having some occasional 'artifacting' on my HP laptop, and screen savers not starting automatically on my HP desktop. Both of these issues have been fixed by automatic updates, and I can honestly say that I've experienced absolutely no bugs aside from these two minor issues.
As far as performance goes, my HP desktop is 7 years old, running an Athelon 2600+ with 1GB of RAM and an IDE hard disk (the original drive that came with the computer). Astonishingly, Windows 7 performs flawlessly even on this very old hardware, and with all of the Aero effects turned on. Both of my laptops have 2GHz Core2Duo processors, with the HP having 2GB of RAM and the Dell having 4GB of RAM. Performance on both laptops has been absolutely stellar. Startup & shutdown is extremely snappy. Applications open quickly, and the entire experience feels very responsive. I can tell you that performance on similar hardware feels faster than XP, even with all of the effects turned on, so needless to say, 7 blows Vista out of the water when it comes to performance. Even Openoffice, which is notoriously slow to launch on Windows, launches and performs reasonably quickly on Windows 7, something that cannot be said for its performance in XP and Vista. Microsoft's Internet Explorer 8 also feels significantly faster in Windows 7 than it did in XP or Vista.
Useability on Windows 7 has kept many of the same tweaks as with Vista while adding more onto that foundation. The search features of Vista are still in place, and still built into the start menu, and search performance has increased drastically. Microsoft has done away with their traditional organization of data (my documents, my music, etc) and has upgraded to 'libraries', which allow you to use folders from multiple locations, including network locations, to comprise your documents, music, and other user folders into one cohesive, searchable location. Homegroups make the creation and use of a home network incredible simple. Setup takes only a few clicks, and discovery of network resources is automated after that. In my experience, the homegroup feature functions beautifully and simply. Windows 7 is also the first desktop operating system to natively support Multitouch, and from what I've seen in demonstrations, it works very well and is very intuitive.
The multimedia functions are simply stellar, and with a quick download from Windows Live, you can add MS's photo & video editing programs to Windows 7. The improved Media Center is both slick and functional for either desktop media use, or for a home theater system. Multitouch capabilities (which I have not tested myself, but have seen in action) only add to the draw and 'wow factor' for the Windows 7 Media Center.
Overall, I think Microsoft is delivering a solid OS that is a worthy successor to both XP and Vista. Windows 7 is a truly capable operating system and is a formidable contender in the PC market.