Do you have a computer? Do you have fingers? Do you type or game on that computer using your fingers on a keyboard? You should get a mechanical keyboard.
The vast majority of mechanical keyboards on the market today (aside from Model M-alikes) use Cherry MX switches. These switches are referred to by color, e.g. Cherry MX Reds. Each color has slightly different action, and this guide at Overclock.net is the canonical reference for such things.
In my experience, the type of mechanical keyswitches matters less than the fact of mechanical switches--compared to the dome-membrane of cheap desktop keyboards, or scissor switches of most laptop keyboards, they last longer and feel better to type on, and you can press more keys at once. Plus (depending on the type of switch) they're actually easier to use. The only hitch is the cost: you're looking at around $60 minimum for a mechanical keyboard, like one from Monoprice.
The difference between typing on a mechanical keyboard and using a standard dome membrane keyboard is like the difference between running on a track in running shoes and running barefoot on sand. And no, I don't want to hear about your minimalist running shoes or your Vibram FiveFingers. But silly metaphors aren't the real reason to buy a mechanical keyboard. Here are the real reasons.
They're durable! Because each key uses a separate mechanical switch, rather than a single rubber sheet with a bunch of contacts on it or a bunch of little flimsy scissor switches, a mechanical keyboard will last a lot longer than a cheapo keyboard. There are plenty of original IBM Model M keyboards with PS/2 connectors that are in their third decade of service. Plus, on the off-chance that you actually wear down a keycap, you can replace it.
They're clicky! You can always tell when someone is typing on a mechanical keyboard. While only some of the switch types have actual "clicks" or "bumps" that indicate when a key is activated, most mechanical keys make a clicking or tapping sound when they bottom out, which goes a long way toward making you feel like you're getting actual work done, and can be reminiscent (for older folk) of the noise of a typewriter, or an old computer keyboard. It sounds like you're accomplishing something. I'm not saying this is completely missing on newer keyboards, but some immersive writing applications even add in typing noises to replicate the sound of typewriter keys. You don't need to do that if you have a mechanical keyboard.
They're more precise! Since there's a several-millimeter valley between each key cap, and the caps are curved, it's easier to hit the key you're looking for (and not the one you aren't). Also, because they have longer travel distances, it's a lot easier to tell when you've activated a key or not. If you want to know for sure, you can get one of the clicky or bumpy key types and get actual tactile and audio feedback for each activation--useful if you often hit a key without bottoming out.
You can do more things at once! You can press more keys at once on a mechanical keyboard than you can on a rubber-dome keyboard. A PS/2-connected mechanical keyboard can have true n-key rollover, while those connected via USB can only register ten keys at once: six alphanumeric keys, plus four modifiers--which is fine, because most people have ten fingers at best.
Most non-mechanical, USB-connected keyboards have limited key rollover, especially for keys near each other.
This is not really important for typists, but it becomes very important for gamers.
Most non-mechanical, USB-connected keyboards (which includes laptop keyboards, since they're usually on an internal USB connection) have limited key rollover, especially for keys near each other--often, pressing two or more keys in a cluster will cause other keys in that neighborhood to stop responding until those keys are lifted.
Microsoft's Applied Sciences explains: “Most keyboards are made of a stack of plastic sheets printed with silver ink in a grid of column and row wires, initially unconnected, underneath the keys. A key press can then be detected as a connection made between a particular pair of column and row wires from the pressure of the key above it.”
This columns-and-rows design isn't a problem if you're pressing two keys--even if they share the same column or row wire, the software can tell which contacts are being made. But if you're pressing three keys that share columns and rows, the keyboard can't always tell which combinations are being used.
Christopher of the blog ControlSpace explains that, for example, on the Microsoft Sidewinder X6 keyboard, you can't press Ctrl, W, and R at the same time. Sorry, crouch-walk reloaders. "Whether you are a gamer, a photoshop user, or power user of other software you may come across certain 3-key combinations/shortcuts that may not work." Granted, many non-mechanical gaming keyboards are now wired so that keys near the WASD cluster have greater rollover than keys elsewhere, to prevent embarrassing jams like that.
How to Test Your Current Keyboard
There are several ways to test your keyboard's rollover capabilities. The first is with a simple text input test on the ControlSpace blog, and there are several standalone programs linked there as well. But he also links to Microsoft's anti-ghosting test, which you can take below. Try pressing common gaming combinations, especially around the WASD keys. You may be surprised at how many combinations just plain don't work.
Which brings us back to mechanical keyboards. Each switch is its own circuit, so you can press as many keys as the connection itself can handle. On a PS/2 connection, that's all of them. On USB, that's the aforementioned six alphanumerics and four modifiers at once. As ControlSpace points out, the only real disadvantage of PS/2 is that it doesn't hotplug, so you'll need to restart your computer if you add or remove a PS/2 keyboard. And your motherboard may not have a PS/2 port. If not, a USB mechanical keyboard is still better than a non-mechanical keyboard.
Mechanical keyboards aren't perfect for every scenario. That lovely clicking noise as you type may end up annoying your coworkers, if you work in an office where people take frequent phone calls. When I worked at Maximum PC, Mike Brown briefly used a Das Keyboard with Cherry MX Blue keys. Sitting next to someone typing quickly on Cherry Blues is like sitting next to a concrete stairwell into which someone has just upended a box of Ping-Pong balls.
Which is to say, it's amazing.
So What Keyboard Should I Get?
Great question! It depends what kind of key switch action you like. There are a lot more mechanical keyboard options than there were a few years ago, and it largely depends on your aesthetics and the kind of switch you prefer. Overclock.net has a long, long list of recommended keyboards sorted by switch type, and this buying guide from /r/mechanicalkeyboards is good too.
If you don't know what kind of switch type you like, you can buy a switch tester from Cooler Master for about $12. It comes with six different switches: Cherry MX Blue, Red, Green, Brown, Black, and Clear, and also a $15 coupon for a Cooler Master mechanical keyboard. It doesn't have Alps, Topre, or buckling-spring keycaps (the other, less common mechanical keyboard switches), but no tester I've found does.
I personally use a SteelSeries 7G that I've had for four or five years. It uses Cherry MX Black keycaps, which I prefer--I like a non-clicky switch with a decent required activation force; I don't want to accidentally press a key when my hand is resting on the homerow. I've had that happen with Cherry MX Red switches before. I find the Cherry MX Blacks to be a good choice for both gaming and typing.
Regardless of the switch type used, I'd go for any mechanical keyboard recommended by the guides above over any dome-membrane or scissor switch keyboard. It's just a superior experience in every way.
Readers: What's your mechanical keyboard of choice? What switches do you use? Any tips and tricks you've picked up over the years?