Such a course really requires a full-out effort and is, in that respect, easy to pace. It hurts like heck, but is not very challenging in other aspects. It's easy to find and maintain your rhythm and just "go with the flow". I'm a firm believer in this type of training, since it teaches correct pacing and develops a feel for how hard you can go. As long as the course is short enough, you recover quickly from it and therefore you can safely do it at least once a week. Combined with traditional interval training, you will build a good capacity.
At a less frequent interval I run through a longer, 50K course. It's rolling and exposed and a great challenge. Since it requires longer recovery times I might only run through it once a month. As for a training effect, it's not much more effective than the 20k course - but it helps as a performance gauge.