The comments given here are valid. A lot depend on the reasons for getting a monitor calibration tool. I think you need to understand how monitor calibration came to be and why it is crucial to certain group of users.
Most people don't need one especially with good modern monitors we use now. I use my Dell U3011 directly out of the box with just a software calibration from one of my graphic design software and I am ready to go. Before you consider a pricey item like a monitor calibrator which only gets used at most a few times in a year if ever, a good quality monitor is a better investment first. Read up on good monitor use for graphic design is a good place to start.
These days, I present my photos or artwork digitally via the monitor and online (webpage, facebook,picaso etc) INSTEAD of doing printed media work like before. I use to be an art director/designer so in the early days a monitor calibration tool is for all my monitors at home and office to ensure the same photo or artwork file I am working on is seen in the same accurate color rendering. This is when calibration monitor came onto the scene. Crucial colour depiction for advertising work for example is important when the artist import the photo into the computer and view it on their monitor. Base on what they see, they cut, paste,adjust, edit the artwork before finally preparing the final version for print ads, poster banners...etc. Colors range presented on different make and types of monitors was an issue back then more then today. Back then it went beyond just buying the calibration tool, the room lighting has to be of a certain level thru out the day, the monitors has to be switch on for about 30-45 mins before any real graphic work is done on it to ensure the CRT tube is warm up to product consistent colors! And also the kind CRT or LED ...eg monitor that was used in my earlier days is far more varied so color calibrating is very important as some hues can come out looking different from one monitor to another. Anyway.. just a little history lesson on how the calibration tool came about.
Don't be "over sold" on the idea that all photographers MUST need one (like other topic that say must get this lens as a Pro or this body or this accessory blah blah blah). It really depend on what you do with it on the long run unless money is no object to you for just an incremental improvement. If all you are doing is presenting your photos online, like another forummer mentioned earlier, your monitor may be calibrated but the thousands of monitors tuning in to your photos may not so how?
The good news is the variations of color difference between yours and theirs will not make that much of a difference with most modern monitors. Color shift will almost never happen unless you did some really terrible over Photoshopping of the colors and that could be more your problem then even a color calibration tool can help you. remember, the tool can help you ensure your monitor to calibrate to a certain universal standard but if you are bad at adjusting colors and over did it.. you are on your own with that boo boo.
One particular group of photographers I know will want a monitor calibration tool are wedding photographer because they have to make ink jet prints or printed copies for their clients. If all you do is view and present your photo works via the computer screen, you really don't have to have one.
There are those of us who use our eyes to calibrate our monitor but that may takes time to get a good eye for it doing it and there are also software that help you calibrate your monitor to an acceptable level.
