Dedicated fans of the Smiths will disagree on whether "How Soon Is Now?" is truly indicative of the Smiths' soul and style; many believe the song is more "mainstream" and commercial than the band's best and more representative work.
Ironically, though, it's the band's calling card. It's their most well-known song (at least, to non-Smiths fans), and it continues to be recognized by critics and list-makers. For example, it's ranked #477 on Rolling Stone's list of greatest songs, #28 on Q magazine's list of 100 Greatest Guitar Tracks, and #72 on Blender's list of the "500 Greatest Songs Since You Were Born."
Even Morrissey and Marr have suggested that the song isn't their favorite, but Marr in particular has to accept responsibility for its mainstream popularity. Marr said that he set out to produce a classic riff, one as universally recognizable as Derek and the Dominos' "Layla." And in building the riff, he drew upon some pretty mainstream influences including disco and Bo Diddley.
Moreover, when the song was completed, they all realized that they'd produced a song with mainstream hit potential. Producer John Porter said that he believed the song would turn the Smiths into a band with an R.E.M-like appeal.