On the UNSW Open Day, I asked the Law professor how many people do the BE LLB and he said there was like around 6 students, so it's not that common and I think it would be safe to assume that those students are great time managers as well. But he did say that you'd be in great demand because of your (apparent) skills in technical, analytic and decision making.
In the short term, BE LLB is just as useless as BE BCom and BE BA. BE LLB is 6.5 year full time (from 2012). A BE is 4 years and LLB is 3 years, so you don't really benefit from the combination, unlike the other combinations where 1.5-2 years are cut down. They would only really complement you if you intend on going into management, patents, HR, higher end contract signing etc.. In all seriousness, if you do intend on going into those facets of engineering, experience is key, so whilst you may have that LLB degree in the bag, you won't be using it until you have experience and probably even a CPEng for larger companies. Because you don't really benefit from the combined degree, you may as well do a JD later on if you do want to go into said careers, or a MBA for management.