Here's what TheCoffeeBrewers has to say:

When you prepare coffee or espresso, you need to be aware of the mineral content in the water that you are using. Since the preparation of (American) coffee and espresso are predicated on very different extraction techniques, the "best" water is different for coffee than it is for espresso.

To review (or in case you weren't aware), the flavor in coffee is mostly contained within the oils within the beans. Brewing coffee or espresso is a matter of extracting these flavors from the beans (the coffee grounds) so that they permeate the water.

The preparation of plain coffee is a steeping process, almost exactly like tea. The coffee grounds (coarse grounds work better for plain coffee) are mixed with near-boiling water. The heat and minerals in the water work together to extract the flavor from the coffee. After a short steeping period, the grounds are strained out of the mixture (via a filter), leaving the beverage known as "coffee."

To get a flavorful coffee, there must be mineral content in the water. If the water is distilled, or if it has been softened too much (softening is the process of removing minerals), the extraction will be weak, and the beverage will be relatively flavorless-as food can be if no salt is used.

On the other hand, espresso extraction is a very different process that does not require minerals, and in which near-boiling temperatures are actually detrimental. For espresso, a more finely ground coffee is first compressed into a "puck" through which water will not pass easily. Ideally, immediately prior to extraction, the puck is pre-wet (both to begin dissolution, and to make the density within the puck uniform, so that the extraction will also be uniform).

Then, hot water (195-200 degrees Fahrenheit) is rapidly pushed through the puck under pressure. Ideally, the pressure should be in the 10-15 bar range (1 bar = 14.6 pounds per square inch), and the extraction time should be 20-25 seconds, maximum. (A longer extraction will result in a bitter and burnt flavor.)

In this kind of extraction, since the water is forced through the puck very rapidly (each water molecule moves through the puck in a fraction of a second), the water is not in contact with the coffee long enough for the minerals (in the water) to play much of a role in the extraction.

Also, for those of you who have taken some Chemistry, you may remember the ideal gas law: PV = nRT. While we are dealing with fluids in this case, note that Pressure (P) and Temperature (T) are on opposite sides of the equation. Since we do espresso extraction under (relatively) high pressure, we do not need a boiling temperature.

In fact, water that is too hot will over-extract the espresso, resulting in a bitter flavor. The reason that moka pots (stovetop brewers) tend to make bitter brews is because the temperature is steam temperature, and the pressure is too low, so the extraction will tend to be too long.

Therefore, minerals in water will not enhance the flavor of espresso. On the other hand, minerals will build up on the inner surfaces (the boiler, the internal tubes, and the portafilter) of the espresso machine. This buildup will alter the pressure within the machine, and it will corrode the internals of the machine.

The gradually increasing pressure change will adversely cause the uniformity of the extraction (hence, the flavor) of the espresso to change over time. The added pressure will also cause the internal pumps and gaskets to wear out quickly. By far, the one thing that is most detrimental to an espresso machine is mineral buildup.

This is why it is so important to do periodic cleaning of an espresso machine (as per the manufacturer's recommendations) with a de-scaling agent. In addition, it is best if you use distilled water, or at least a water softener. For commercial machines (which will see heavy use), an in-line water softener is essential.

While drip coffee-makers will also get mineral buildup, and should be de-scaled occasionally, this is just so that the water will flow (at all) through the machine. Since no pressure in involved in the brewing of drip coffee, mineral buildup will not damage a drip coffee-maker the way that it will destroy an espresso machine. If you have an expensive espresso machine, it is imperative to keep it clean.

For plain coffee, a minimum mineral content of 150-200 parts per million is essential to a good extraction. Water softer than this will result in weak and flavorless coffee. For espresso, you should use distilled water. If the espresso machine is connected to the building plumbing, an in-line water softener (to remove the minerals) is essential.