This is a classic Sorkian flashback episode. Jumping between Matt's quick decline into drugs and his memories of first meeting Harriet back in 1999. Not only is it great drama but Sorkin also captures the differences between the two time periods. A point in time when our biggest question wasn't about Y2K.
What made this episode so intriguing fantastic is that the audience knows, that both writer Aaron Sorkin, and Matthew Perry, have struggled with substance abuse. Their personal experiences add to the emotional core and truth that permeate throughout the episode. The expositional element of the show doesn't feel as such. So much more validity and depth was given to the Luke-Harriet-Matt triangle. Matthew Perry does a phenomenal job at portraying the 1999 version of his character as optimistic bundle of energy, contrasted with his 2007 version of his character which is pessimistic and depressed.
Overall this should stand with Sorkin's best like "In Excelsis Deo," "Two Cathedrals," and "In The Shadow of Two Gunmen"