I LOVE a good espresso and it always troubled me about the lack of knowledge some people had about coffee and espresso, especially in the U.S., because that's where I am from, I am not sure this is prevalent throughout the world, but there are some things everyone should know about espresso.
First off, it doesn't matter what type of roast or beans you use really. It just matters that the coffee is ground to a certain fineness. This causes the massive amount of pressurized extraction you get from brewing it in an espresso machine!!
The massive pressure that an espresso machine provides is the key to good espresso, not the beans or the blend, or the roast shade. In fact, some of the most powerful espresso I have ever tasted is from a roast that most people would describe as light roasted coffee!!
You see the way the espresso interacts with the brain is very different. It calls into attention a brain function called Adnosine which releases adrenaline into your system along with dopamine, which anyone who has a smoking addiction would know about.
So anyway the result is a rush to your system not unlike an amphetamine which basically speeds up your body and wires you. What an amazing thing this little agricultural product can alter your mind, body and behavior.
The other thing is there are only really 2 types of beans in the world. They are Arabica and robusta. Arabica is usually higher grown and more expensive, yet they have less caffeine in them. They are considered based on grade to be the top of the line coffees, robustas are lower grown wicked stepsister's to the arabica, however they can retain up to 50% more caffeine than their higher grown family. They are much cheaper and yet many companies prefer to roast and sell espresso with robustas so people get the jolt they are looking for!!
Just remember you are paying for a premium product with a less than premium bean!