While I don't enjoy this place quite as much as Tazza D'Oro as a hangout (it's smaller, for one), the coffee and espresso are terrific - some of the best in Pittsburgh (along with Tazza D'Oro and Big Dog - I've not tried Aldo yet).
As other reviewers have said, 21st Street is for serious coffee lovers - the staff is conscientious and makes good recommendations, and whatever they make is made very well.
The Clover machine produces a terrific cup each time, and they also have a fine selection of delicious tea.