You might ask why this post, with such a title, is under the category "Advocacy". Well, if you're reading specifically this category it looks like you're after some persuasion to switch to GNU/Linux. Then there is something you should know about GNU/Linux as it currently stands, and hopefully it won't put you off.
Linux is the ignored cousin of Windows when it comes to hardware support. Out of the box, a Linux distribution supports a huge amount of kit, and Ubuntu supports more than any other distribution. But Windows has hardware support from manufacturers - so any device you buy will pretty much work on Windows as long as you install the driver. You'll have to give up impulse-buying of things, at least until Linux compatibility is actually printed on boxes. You might not be able to buy the device with the best features, and if are, then you might not get access to all those features.
This is even more so with software.
And let it be said that the same applies to Mac OS X, but in a lesser way. Really want that latest Nokia smartphone? It probably doesn't have OS X software for syncing, although there might be something unofficial out there. If you want a brand-new iRiver or Creative MP3 player (fat chance if you're a Mac owner, but still), you'll have to wait until the 3rd-party software for those players is updated to be compatible with the newest models.
With open-source software, it is possible to do lots of things that you can do with proprietary software on Windows. But right now you might not be able to do EVERYTHING. A year ago, I couldn't even encode video properly on Linux, though others could. Now I can encode perfectly and even edit video, thanks to recent advances in the software, but I still can't make DVDs that play reliably. Blu-ray and HD-DVDs can be played on Linux, but it's not as easy as you'd hope.
Prepare to give up some tasks on Linux.
But the good news is that things are getting better. Video encoding software is better now, and I no longer have issues with A/V sync. The advent of Kdenlive makes it possible to do some basic editing and encoding. There are now quite a few playable Linux games, and even Aleph One is now easy to install on Ubuntu (it originally wasn't!). On the proprietary software front, Skype supports video (though not HD video), Flash is as good as the Windows version, codecs for everything except HDD camcorder video are working well, K9Copy has many features from DvdShrink, and Wine continues to get further DirectX support.
This is a good time for Linux users, and it will get better thanks to the efforts of thousands of brilliant programmers and millions of users. But if you're thinking of switching to GNU/Linux, keep Windows around in a dual-boot setup, on another computer, or in a virtualiser; until you know that you can live with GNU/Linux full-time.
I may curse that I can't make reliable video DVDs (well, not yet anyway), and I have given up a golf game, a trucking game, an ambulance-driving game, and free 3-minute calls to anywhere with Gizmo (it never worked on Windows anyway, to be honest). But it's better than the alternative - an operating system that doesn't make sense to me, is prone to all sorts of threats, is poorly programmed, is an immutable "black box", leaves me to the mercy of proprietary software developers, leaves me to the mercy of bugs, doesn't have the workflow and timesaving features that I like, and costs a bundle.
Just make sure you see what you're gaining, as well as what you're losing. After all, Windows will never become FLOSS, and I'm sure it won't become unrestrictive to its users either. But GNU/Linux, with help from open-source and proprietary developers, will definitely close the gaps, so some day you won't have to give up anything to make the switch.
The sooner you start switching, the sooner you will finish switching (becoming purely GNU/Linux). Take it as slowly as you need to. You will know when the time is right to get rid of Windows/Mac OS. I know: You're saying "How can I ever switch over completely? I'm tied to Windows! If I adopt Linux, I'll dual-boot for the rest of my life!". I know that's what you're saying, because I originally said it. Now I'm running Linux as a single-boot. It took two years to get to this point, and I did still have to give up a couple of things, but it's worth it.