Ah, yes, the "Long A." I first learned it as part of my effort to master that fingerpickers test-of-manhood/rite-of-passage, Mississippi John Hurt's "Candyman."

The Long A is not nearly the most difficult chord in that song, though ~ that distinction belongs to the inverted E7 I've heard called "The Spider Chord": 020135, if I'm not mistaken.

A nice riff based on Long A, when employed as the "V" chord in key-of-E blues, is to use the ring finger to alternate between 002225 and 002425 (and 002223, to, if you have it in ya).