Windows 7 is a pretty solid operating system... for Windows.
I've been using it since the first release candidate. Running the 64-bit version on my Hackintosh machine (meaning that the Windows 7 I'm running is more "native" or natural to the system than my two Mac OS X installations). Despite that reality, my Mac OS X installations have, in general, been far more stable and responsive than Windows 7, despite being a "hacked installation" (retail DVD installation using the BOOT-132 method). I find Mac OS X is still much more enjoyable to use, more reliable, more generally stable, and more conducive to efficiency than Windows 7, even when on a Hackintosh.
In spite of all that, Windows 7 is much, much better than Vista. In fact, were it not for Vista's craptacularness, Windows 7 wouldn't seem nearly so great. Besides, Windows 7 is moving eerily close to what Mac OS x was (and is). The "Aero" look and the new Windows 7 stuff is reminiscent of Aqua and the "bubble icons." The new option to "pin" things to the taskbar feels very much like it's becoming more Mac OS X Dock-like. The calendar program is terribly similar, and the search feature feels like a crappy version of Spotlight that doesn't work the way you wish it did. Unfortunately, that's as far as the similiarities go. Windows 7 still carries the baggage of being built as Windows (DLLs, registry, ActiveX, IE, IE integration, system tray garbage collection, viruses, spyware, etc).
And I'm amazed that it *STILL* suffers from glitches that Mac OS X has almost universally left behind. Awkward things like long pauses when you launch an app, an application's Window being slowly drawn from top to bottom when switching from a large app to another, minimized full-screen apps failing to come back up, full screen apps "accidentally" forcing a system-wide resolution change that you must manually fix, system tray apps going unresponsive, crashed applications crashing the entire system, etc.
Additionally, "sneaky" updates that seem to hit you with a "You must restart now" dialogue are painfully annoying. I swear I turned those off, yet there they are again. And that's enough to be very annoying, were it not for random other things occurring. Torrents barely move in 7 when in Mac OS X on the same machine with the same program using the same connection with the same settings, they blaze. Some websites occasionally hang endlessly in Firefox on Windows 7 at times, but I pretty much NEVER notice it when running Leopard. When I download unusual formats for compressed files and such, I often have to seek out a program to open it, when in Leopard something built-in can do it for me. Windows 7 also has the virtually useless Aero Peek, which allows you to peek at your desktop, but as soon as you attempt to manipulate anything on it you lose that "peek" and the other windows come back.
Other features I hate include windows maximizing when you move them to certain spots (I don't want it maximized, I want it where I moved it!!), the cursor occasionally getting "stuck" on a window when I move it, requiring that I unstick it from the window by clicking around, the way programs still dump their icons all over the desktop by default, the way some programs seem to fail to put icons in the Programs menus, the way programs tend to want to install garbage (bloatware) toolbars, system tray icons and such on your system, and the Control Panel that keeps looking significantly different with each new system, making it hard to find things that were previously easy. I also don't like how you can't uninstall more than one program at once using the Uninstall a Program from Control Panel.
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I have extensive experience using a variety of operating systems, the most limited being Linux. I've been using Windows and Mac OS for a long time, and I can say that since Mac OS X, I believe the Mac side has been a hands-down winner on a variety of fronts: ease of use, user friendliness, efficiency, stability, and convenience. Additionally, it's subjective to a degree, but I believe my Mac OS X system runs significantly better on the same hardware than my Windows systems ever have. I just feel that everything is snappier and more responsive when I'm using Mac OS X, and that is a feeling I am acutely aware of virtually every time I use one system and then switch to the other. Additionally, I get much better FPS results from the same game running it in Mac OS X even with *higher settings* than when running it with lower settings in Windows. Remember, all of my hardware was built for and designed to run Windows.
Overall, here's what I would score the new OSes:
Snow Leopard
User friendliness: 9/10 (I can think of few improvements, but would like to see some things go away.)
Stability: 10/10 (Running Mac OS X on my Mac computers, I can count the system crashes/freezes on one hand for over 6 years and 5 computers).
Efficiency: 9/10 (With no differentiation between the wallpaper/Dock of different Spaces, I docked it a point. Things like Exposé and the ease of typing accented characters like the one I just wrote are second to no other operating system).
Responsiveness: 8/10 (Apple has made leaps and founds making the system more responsive, snappy, and fast. I hope they continue to push for it, because I don't feel it's quite there yet).
Windows 7
User friendliness: 7/10 (The Control Panel is not straightforward, and crap in the system tray, Program Menus, and Desktop can add up and make it confusing and disjointed. Errors are still vague and unhelpful, and constant nag screens can scare and confuse the computer-illiterate).
Stability: 5/10 (I would never be satisfied using Windows 7 for any extended, heavy work, such as HD video editing. Many do and are able to, but my experiences show that it's generally not stable and produces a variety of problems).
Efficiency: 7/10 (I can get work done, but I wouldn't want to. Moving between windows, or seeing what is in every window is still a chore. I don't see a built-in multiple desktops option, either.)
Responsiveness: 6/10 (Much, much better than before, but there are still issues of slow-down, crawling, slowly drawn windows, and delayed input response).