Class D's are generally around 75% efficient, and stay close to that at all power levels.
Class a/b's are typically around 60% at full volume, but decrease as power decreases to around 30% at 1/3 volume.
Class D's are often intended only to drive subwoofers as many of them do not sound good at higher frequencies. Recently, however, some class D's are being produced that will operate well as a full range amp (alpine's PDX's, Lanzar's "mini-digitals", Eclipse, Blaupunkt)