You've asked a fascinating question, and I'll be *very* interested to read the replies!
Wikipedia offers a good introduction : http://en.wikipedia.org/wiki/Espresso_Machine
I've owned several brands of espresso machines through the years.
I must confess a fondness for the kind that grind whole beans rather than use pre-ground coffee
I currently use a Saeco "superautomatica FAMILY". It works just fine *except* it leaks and has to go to the repair shop twice a year :-(.
My experience altogether with these machines is that they are very fragile and need lots of after-sales service. Of course, even if the machine is guaranteed, you're minus one machine all the time the thing is in the shop :-((((.
The Nespresso solution is ideal for people who make only a couple of cups a day, but is impractical for people like me because the cost of the capules is super-expensive...
All the best,