Not all espresso machines are alike. In Italy, many espresso machine manufacturers have a regional dominance because establishments typically choose machines based on their brand's service area. If an espresso machine goes down, an entire day of business can be lost.
In San Francisco, espresso machine brand and service availability are not significant considerations. Even so, there's no substitute for both good equipment and good equipment servicemen. If the equipment and its service people (macchinesti, in Italian) cannot ensure the proper settings for water flow, pressure, and temperature, the espresso will suffer.
It's difficult to determine cause and effect - whether good espresso comes from good machines and service, or whether establishments that invest in good espresso tend to get better machines and service. However, we've found a particularly strong correlation between good espresso and the use of machines from Faema, Elektra, and La Marzocco. (The cafés we've found with the best espresso in North America almost universally use La Marzocco machines.)
What makes a good espresso machine? Temperature control for one. If the machine runs too hot - espresso should be brewed at about 204°F, just below the boiling point of water - the coffee can be scorched. Too cool, and the resulting espresso can taste sour.
The ability to generate controlled, high pressure for extraction is another important factor for a quality espresso machine. Espresso is best produced under about 8.2-9 atmospheres of pressure. The right pressure is a balance between extracting the right flavors of the ground coffee and restricting the wrong flavors over a 20-25 second pour. Under-extracted or over-extracted espresso can otherwise be the result.
For example, most home espresso machines fall flat (Krups, anyone?) because they can neither generate enough pressure nor can they properly hold a consistent temperature just below boiling. (The lack of a high-quality burr grinder is also a major factor in the failure of home espresso, but fortunately almost all cafes use a commercial grinder capable of grinding beans to the fineness and regularity required for good espresso.)
Espresso machines come in a variety of forms that reflect a past century of technical innovation. In some of the older machines, pressure for espresso extraction was generated by manually pulling a lever. Most commercial machines now have an automatic, electric pump that takes care of this task (a.k.a. the semi-automatic in espresso machine jargon). The difference is akin to the difference between a manual and an automatic transmission on a car. The automatic pumps produce an even, consistent pressure - though there are some purists into the zen of espresso production that prefer the feel of manual control.
Most recently there's been an appearance of super-automatic machines, which can take care of almost everything short of blowing your nose at the push of a button. Many of the better machines of this type are of Swiss engineering, and they've provided great relief from repetetive stress injuries (RSI) for many an overworked Starbucks barista. Their downside, however, is that while they produce espresso of a regular consistency, the overall quality falls short of some of the more manual techniques. As mentioned on the barista page, the best baristas in the world seem to regularly practice quality control - reserving the option to toss out an occasional cup that doesn't meet their standards.