As Intruder says, it does depend on the airplane, my own experience is with the B777, very similar to the B747-400, but in some others I've flown it's not much more advanced than the basic Speed / Mach hold on earlier aircraft.
I agree with the others here that FLCH does offer tighter speed control than VNAV, but the biggest advantage (to me) is that when the pre-planned expected descent path is not achieved, then FLCH is the only practical alternative. With VNAV the STAR, with all of it's speed schedules is planned and executed exceptionally well by VNAV, but the inevitable Radar vectors giving Track shortening / Lengthening and differing speeds can make VNAV more of a liability than an aid. If the STAR and arrival is flown as planned (rare), then VNAV is wonderful. Properly done, and with appropriate SOPs, VNAV is wonderfully accurate for the non-precision approach.
Regards,
Old Smokey