The Rotring Quattro is one of those many-writing-instruments-in-one devices. There are a number of different species floating aorund, but the basic properties are as follows: 0.5/0.7 mm mechanical pencil; black pen; and blue pen. The fourth pen varies across a red pen, an orange marker, and a PDS stylus. In my case, I've got three pens and the pencil.
The quattro is not the smoothest writing pen on the market, what with using standard ball-point. Nor is it the most comfortable for long periods of time--the beautiful metal barrel offers little cushioning. However, that's not why you'd buy it. You'd buy it if you want 4 writing instruments in one, and that one is a beautifully designed and elegantly engineered instrument. For a start, unlike most multipens, the quattro uses an internal gravity-ball mechanism to select which pen comes out when you press the single button at the top. That means you simply tilt the barrel a certain way and a different pen type is selected. It is an intriguing, simple, and mystifying process that attracts all sorts of questions.
As stated earlier, the pen isn't the most comfortable, but it's so useful that I'm willing to overlook that. It's great for taking notes and using different colors for emphasis. It's also just flat out convenient and stylish all in one. Moreover, I've had mine for nearly six years and it still works pretty well. The mechanism does get a little worn with age, but it has never failed. Additionally, although it takes non-standard refills, a number of companies produce compatible ball-points in a variety of colors, making it fairly easy to replace used pens. As for the pencil, it's also pretty robust and the lead is available anywhere.
Don't get this if you want a smooth, highly comfortable writing experience.
Do get this if you like the utility of 4 pens in one combined in a well engineered instrument.