Personally I think as long as we sleep when we can and get some sort of sleep at some point it can't be that bad. I know they say that after not sleeping your brain functions are reduced, and reaction times are slower, but I think over the time I've been doing lonh haul that I've adapted and I feel just as alert coming off a night flight from Singapore as I do if I've woken up after a nights sleep.
When I first started it was a different story, I was no use to anybody after coming off a night flight, but I believe that your body can adapt to it!
Obviously it's not all that great for you though!