Negligible increase in workload. You have to frequently compute estimated track miles to touchdown and make sensible adjustments to rate of descent to constantly watch you don't end up flying level. Very difficult to quantify any fuel savings (if any), but the environment benefits of not levelling off at 3000' over a city and pouring on power are too important to be dismissed. They are merely an extension of proper airmanship. I have sat and watched pilots cleared to 4000' 25 miles out descend 1000 or 2000' with idle power and then level off again! I feel disgust when I see flying like that, from a passenger comfort point of view and noise perspective. They should be mandatory.
There are different regulations in the US where continuous descents should not necessarily to be followed.