We have unisex toilets at our secondary school. They were re-built during the last summer holiday, so we are almost at the end of the first year.
It's really worked. The kids were initially a bit
about it, but all said, it's been fine.
There are two rows of cubicles on either side of a big font style wash basin. One row has the girl symbol on the doors, the other has boys, so really, they are still separate. The only thing they share is the sink.
Staff do not share these toilets. (Unless it's INSET and the kids aren't in) It's not appropriate or sensible to put teachers and students in the same place, even if there is no where to hide iyswim!
As someone else said, it's no different from when they go swimming. In fact, slightly better, because they've got more clothes on! 

From what I've seen, the kids have adjusted very quickly and it doesn't phase them at all. It's only the parents that seem to have a problem with it and I would imagine that's because they haven't experienced anything like it before.