[Stephen]

Sevenths add flavour that you don't get from plain major chords. One of the starting points I use for finding new progressions is the circle of fifths (there's a post in this forum under that name) which works best with sevenths.

The chord progression came partly from playing around with the C-Cm change (a trick I learned from looking at "September Song" a jazz standard), partly from the snatches of melody I had floating around in my head and partly from applying the circle of fifths idea.