You should probably buy Windows 7 if you use Windows, that much is clear. Microsoft has finally given their users a good, modern operating system. The best of what's new that I've come in contact with in the little amount of time I've used it are the many changes to the interface. This makes me happy, as the UI of my Macintosh home has't been exciting in the past few years. I enjoyed discovering the little pieces of candy Microsoft built into 7, even though I remain more comfortable in OS X.
That said, there are a few adjustments worth making. These are the ones I made within the first day or so of messing around with the 7.
ONE: Turn UAC from a problem into a featureNo, they haven't gotten rid of UAC, also known as User Account Control or those incessant cancel/allow dialogs. You are still going to have to click away a little dialog box when you install something, though a few of the weirder things that would set off UAC warnings in Vista have been disabled by default.
While I understand the problems people have with it, I don't really mind UAC. It's comparable to dialog boxes in OS X that ask you for an administrator password when you work with sensitive stuff. The only reason it's a problem is because you can't piss in Windows without needing admin access, and until Microsoft radically adjusts how their OS works they've made exactly the right decision with UAC.
Still UAC is worth toning down in Windows 7. Here is how you do it.

In the taskbar there is a little flag icon they call Action Center. Click on it to get a Control Panel window that it shows a lot of little bits of information about your computer. This is how Windows tries to make you take care of it with stuff like firewalls and Anti-Virus. Anyway, on the side is a button called "User Account Control settings," click that to get a slider. Move it to either the second setting from the top or the third, both those make sure that UAC only pops up when it's really necessary. The only difference between the two is that the lower one removes the dimming effect that made UAC sometimes seem like it was signifying the end of the world.
TWO: Disabling transparency and other thoughts on Areo GlassJesus Christ hell who thought that making everything see through was a good idea? Look, go to Control Panel and then Appearance and Personalization >> Window Color and Appearance. There is this checkbox.

Uncheck it with a vengeance.
I can't change the fact window titles are nigh-unreadable in a lot of places in Areo Glass, the default theme, and the other things I don't like about it. Still the benefits of keeping the theme the default are worth. The animations are a big one, as they overall communicate the various UI concepts very well.
There are a few that I'd like to see gone, like the animation when you close a window that looks too much like minimzation and the delay in switching that window previews from application to application as you hover back in forth over the taskbar, but I both don't know how to change them and they aren't any worse than not having the new animations at all.
This is kind of unimportant, but there is a way to mess around with Areo Glass animations that is ridiculous in a truly Microsoftian fashion. Buried deep in the Control Panel are some options that allow you do change a few animation options for the sake of performance, though if you can't run the whole shebang you probably shouldn't be using Areo Glass (or hell, Windows 7). It's under System and Security >> System >> Advanced System Settings >> The Advanced Tab >> The "Settings..." button under Performance. After all the work I went through to find that none of them do what I want, but oh well.
THREE: Use Dropbox with your librariesOne of the best features in 7 is also one of it's most stupidly simple. It's called libraries, and they are just a bunch of file locations who's contents can be found under categories like Documents and Videos. There are no indexes to create and manage. Databases like that are great (see iTunes), but they create inflexible file structures and a host of their own issues (ever try to move an iTunes library?). For a lot of things these are perfect, and truth be told they are something that I really want in OS X.

Libraries also work really well with
Dropbox. If you aren't using Dropbox you really should. It replaces in one fell swoop everyday use of flash drives and e-mailing files. But yea, this article isn't about how Dropbox is awesome (this one is!:
Rands In Repose >> Dumbing Down the Cloud), it's about the fact that sometimes you forget if a document is in your dropbox or on your computer itself. Now it doesn't matter, it's all the library.
Documents are the only things that I sync using
Dropbox, but you can use a simular method for other types of files. Simply navigate to the folder that you use to store a certain type of file in dropbox, click the "Include in Library" dropdown box from the vertical menu and select the library that is most appropriate for that box (if you have subdirectories in that folder I recommend adding those too). If there isn't one, create it.
Now all you need to do to find any a certain type of file all you need to do is browse to its library. Lets say it's Documents. If you want a Document to be saved in a Dropbox just drag it from My Documents to the dropbox right from the library, and if you want to get it out of the dropbox to save space just do the opposite.
FOUR: Don't use Microsoft softwareOkay, that's not totally true. Windows Media Player, for instance, isn't all that bad, and if you need it Office is just about the only game in town when it comes to Windows productivity software. Still, you know the drill if you've ever started a Windows installation before: download Firefox,
Google Chrome or something else to browse with other than IE, then get iTunes and VLC to replace Windows Media Player. It's easy enough.
The other things that I downloaded in the first day of running Windows 7 were the previously mentioned
Dropbox,
Trend Micro Antivirus and the game download service
Steam. Trend Micro is the only security software that ever made me happy, and Steam is practically the reason I bought this computer.
You might notice that the only pieces of software I have that are Windows only are my Anti-virus and my games. Congradulations Microsoft, you have officially pushed out all competitors from Windows and turned your OS into a software wasteland.
A continuation! This ends the things that I have to say on Windows 7 that are useful right now. If you want to read some more about the new Windows I've written
What sucks about Windows 7 that I can't change (also some things I like and bringing this thing to a close).