There's more to this than just personal convenience, which really hasn't been taken into account. 9/80, and other more extreme methods (like 12.5 hour days 3 times a week) are actually a fast way to decrease your performance on the job. Psychological studies have shown that extended hours, leading to reduced sleep, tend to have very adverse affects on attention span and reaction time.
A very extreme example of this was acted out at a nuclear power facility, somewhere near (i.e. within blast radius of) New York. This happened during the early hours of the morning in the night shift. A cooling pipe had been plugged by a maintenance crew for cleaning, and they had forgotten to remove it. The monitoring crews noticed that this pipe had very little pressure in it. So, they decided that this could be remedied by shutting off all the other cooling pipes. Now, nobody there figured out that this might potentially not be the problem, and had it not been for the well rested daytime foreman who arrived during the shift change, who noticed this and immediately deduced that a blockage was to blame, well, the population of North America would be significantly smaller.
I know that the plural of "anecdote" is not "data", but it's really necessary to take this kind of thing into consideration. It may sometimes be more convenient to have longer periods of free time, like an extend weekend, but really, making it up by adding more time to existing slots isn't always a realistic solution.
That said of course, I've never done 9/80, so I can't really say how it affects your sleep cycle or job performance. Just please take caution when looking at this kind of thing.