How do you train when you only have 20 minutes?

Most workout routines assume you have plenty of time, but work, family commitments, and travel can sometimes leave only 20 or 30 minutes for training.

When that happens, is it better to do a shortened workout or skip the session and return to your normal routine another day?

How would you structure a genuinely useful 20-minute workout? Would you concentrate on one or two compound exercises, use supersets, reduce the number of sets or perform a quick full-body session?

It would be interesting to hear the shortest workouts members have found effective. Have brief sessions helped you remain consistent during busy periods, or do you find that rushing makes the workout less productive?