Think of the Manhattan Special as the Red Bull of its day, which goes back to 1895. Cold, sweet and intense, this espresso soda fueled many NYU study hours for me. Now available in diet, which does help with the post drink jitters, it still is an acquired taste. It is thick and syrupy, very coffee flavored, sweet and slightly carbonated. Pretty much everything I like in one small bottle. The Manhattan Special makes regular iced coffee seem like it's for sissies.
I've only ever seen this in New York. It might exist elsewhere but to me it's something I can only drink when I'm in a New York frame of mind.
Update: BevMo carries the regular non-diet version.