The music from series 1-3 was top class like the little "Posh" stings in better than life, but the quality slipped with more synthey sounds coming in (To the best of my memorey) it then picked up around series 6/7.
Howard Goodall composed tracks for all Series I to VII, sometimes episode- or scene-specific, sometimes generic for use wherever. Library tracks were also used throughout the show, though much more in Series VII, VIII and BTE. I'd be hard-pressed to hear a significant quality dip in the mid series, as suggested. But since these things are only ever a matter of taste, I guess everybody's gotta have favourites.
