Since there was a little Mac vs Windows vs Linux talk on the last page I'll chime in
Having used Linux, OS X, and Windows, imo the advantages of each system are a lot different to what people usually say. I think it really depends on your needs, technological capabilities etc to which system is right for you.
Linux:
Speed is hands down where this wins. No Mac or Windows machine will ever run as fast as well as a Linux box, you can easily get your boot time down to ~5 seconds even with 3-4 year old hardware, really put your hardware to good use and get the best out of it. Linux machines are also the fastest gaming machines around, when people say Linux machines can't run games, what they mean is that most popular games are built using Direct X and other software that unfortunately only runs on Windows. In reality Open GL is just as capable as Direct X and supports Windows, Linux and Mac OS X, but it'll be a long time before the majority of developers make the switch or consider using both.
Another plus side is the lack of malware for Linux. You've probably heard the saying that Mac is virus-free, but in reality no OS is, just Mac and Linux have fewer malware programs written targeting their OSes due to their userbase being smaller than that of Windows. Statistically, Windows has the most malware by far, Mac comes second, and then Linux has a minute fraction of the malware of either system. Whichever one you use you should always have anti-virus software, firewall etc installed though.
This one also wins out for being Open Source of course, but that also causes Linux's major downside - fragmentation, and I'm not talking the disk kind, since there are so many different distributions out there, the worldwide programming effort for Linux is split up big time, and at the very least it takes a long time to choose which distribution suits you.
Use Linux if: You're tech-savvy and are willing to go through and spend time configuring your settings for a good many hours you'll get a beast of a computer, running at insane speeds, with a little cost reduction over other computers, and free updates for life. Also use if you're not concerned with using the most popular applications but are more than happy with free open source alternatives. It's also worth considering using if you don't game and just want a fast cheap computer for daily tasks, with little maintenance required.
Don't use Linux if: You want something that's at its fastest out of the box, or you are a gamer looking for a wide selection of the most popular games.
OS X:
Speed/usability out of the box is where Macs come out top. The reason you buy your Macs prebuilt and don't build them yourself is because they are already configured for the specific hardware they run on. This is why so many people will boot up a Mac after having used Windows machines all their life and be amazed with how fast it runs from the get go, and how smooth it seems.
Macs also have the advantage of bringing Unix to the masses. Linux is based on Unix but due to the difficulty of setting up many of its distributions and even the easy-to-use ones being fragmented into hundreds of different distributions and versions they all have a smaller user base, less support, and of course, being free and not bundled with your system there's no way of calling up your support line or sending your system in for fixes if something goes wrong - unless you're paying your local technician. Macs on the other hand have bundled Unix into an easy-to-use system that has a massive userbase, and have made a lot of people learn the basics of a Unix system.
The major downsides of Macs are the price and the lack of options for customisation. In the simplest terms, you are always paying too much when you are buying a Mac. You can often price up a Linux or Windows machine, OS included, for as much as $400+ cheaper than a similarly specced Mac, and essentially, with OS X being a well-configured edition of Unix, you're basically paying someone $400+ to set up a distribution of Linux for you and configure it for your hardware, while not getting to choose that hardware. With some Macs though the price gap is smaller, just be sure to compare and contrast with custom build PCs when looking for one. The lack of customisation limits you in speed because you are almost always unable to get the fastest hardware out. Similarly to Linux this also suffers from a lack of game developers who're willing to make Mac versions of games, however that's slowly changing, and Mac has a lot more support for popular games than Linux does, by far.
Use OS X if: You don't want to spend time configuring your system and want things running smoothly from the get go, combined with better support from more popular applications and games, while not needing everything that's available to Windows. Also use if you're not willing to build your computer yourself/get someone else to, prebuilt Windows machines, like Macs, are overpriced, and so if you're not going to customise then the Mac is the better choice, and will run faster and last longer for the same amount of money.
Don't use OS X if: You're looking for customisation. If you're looking for software or hardware customisation options you're far far better off with either a Linux or Windows machine. Also don't use if you're just buying your Mac for Photoshop, Photoshop runs just as well on a Windows machine which will be at a lower price tag, and there are even free alternatives for all three operating systems. Same goes for Garage Band etc, they all have alternatives that run on Windows/Linux, which are usually similarly (or better) fleshed out.
Windows:
Software development base is where Windows comes out on top by far. Windows has the largest market share and as such there are far more developers targeting Windows machines alone and not bothering with OS X or Linux at all. This means more programs (both free and paid), more games, though on the downside more malware. If you're not prepared to run a good free anti-virus program and regularly scan your computer then this makes Windows a very risky option, but if you are this opens you up to a much wider variety of choice than the other systems combined.
Windows is fully fleshed-out with this huge variety of software and games and still affordable. While Linux has low support by application developers and is free, and OS X has medium support by application developers and is very expensive, Windows is rather cheap and offers the most applications of all the systems as already mentioned. This means Windows is really the winner on customisation, as although it doesn't quite have the ability to take apart and configure your OS to a deeper level like with Linux, it goes far further than a Mac and then offers you the widest variety of software, while still working with all hardware. In general with Windows, if someone wants to send you a file or an application, you know it's going to work on your system, whereas with Linux or OS X, this is less likely by far.
The real problem with a Windows machine is that it requires the most maintenance out of the three systems. If you're not willing to scan your machine, set your computer defragmenting, making system restores, backing up data etc you're likely to run into many problems in the long run. This can be tedious and most users would rather avoid all this extra work and just have it done for them. Now with fewer ways to really customise your operating system to the wide variety of hardware you could be running, you're also not going to get your machine as fast as a Linux machine, ever, but on the flip side, while you're not as fast out of the box as a similarly specced OS X machine, with a little configuration you will run much faster - and you beat both systems with the ability to actually use your hardware for all of the latest games.
Use Windows if: You want to be able to use the majority of applications and games available and care about the availability of software, there's really nothing that's off limits to a Windows user thanks to this. Also use Windows if you're a gamer, this simply should be your first choice system, so many more games available, and even the games that have been ported to Linux and OS X will run faster on Windows simply due to the fact that they were designed first-hand with Windows in mind - Windows is the system for gaming really, beating even consoles by far in selection of games, speed at which it'll run them, and price per game.
Don't use Windows if: You don't care about using the big brand applications and games and are happy with a smaller selection of software, as really Windows relies on its wide support for different software to make it win out, and without that would lose out to both Linux and OS X. Don't use Windows if you're going to buy a store-built machine, always pick the parts and build yourself, you're going to be paying the same price for a prebuilt Windows machine as for a prebuilt Mac, only you'll get some cheap no-name brand Power Supply and Motherboard, resulting in your system dying within the next 1-4 years, and running nowhere near as fast out of the box.
Tl;dr: Linux is best for max speed if you have time to customise it, OS X is best for usability and good speeds out of the box and for people not willing to build their own systems, Windows is best for those who want a wide variety of games and applications and are willing to build their own machines.