Sorry but the first comment here is a little ill informed. I'm a commercial barista of 10 years experience and have made hundreds of thousands of espresso based drinks. I have used many brands and makes of commercial espresso machine in this time and the average working pressure of these machines (i.e. whilst pulling a shot) has always been between 8.5 and 10 bars. It is most often (and ideally) about 9.  It is never 15 bars, and if it was it's time to service or humanely put down your coffee machine. Higher pressures cause over compaction of the coffee grounds in the group handle (portafilter), increasing resistance to flow.  The corresponding reduction in flow rate causes over extraction and consequently produces quite a lot of crema but  a burnt taste (not so good). 
 Yes there's lots more to add to this article but better to add it than criticise what's there. All in all I found it interesting and quite informative for such a short piece.  One thing worth noting is that the author seems to be refering to the Italian standard espresso (7 grams grounds producing approx 30 ml espresso shot in around 15-20 seconds...depending on many variables). The trend in the US is (I'm told) toward using double and even triple ristretto pulls to produce single drinks .. so for example up to 20g of dry grounds may be used to produce a single espresso drink (e.g cappucino) 
 One last comment: espresso culture is wonderful.. coffee snobs are not! after being intimately involved in this world for years I can tell you that the best coffee is the one you like. There is no perfect bean, no perfect blend, no perfect grind. It varies depending on everything from your mood to the time of day. Costa Rican arabica tastes completely different to Guatemalan arabica but both have there place. The same applies to mokas and french presses, arabica v robusta etc.  Learn the art of using each tool to make the best possible coffee and learn to appreciate the different characteristics of each type of coffee. It's easy to be a self proclaimed coffee geek (what others call a coffee wanker) ... harder to be a person who really appreciates coffee in its many varied forms. Nevertheless its nice to see people taking such an interest in the fine art of espresso. Just be careful in criticising the opinions and research of others. It's easy to flame, harder to learn.