Hampton, yes, we had a Saturday interview at T for DS1's 10+ place (he is now a very happy 3rd year there, so places are definitely offered from Saturday interviews!)
There will be lots of boys all being interviewed while you are there, so it will seem very busy with lots of parents anxiously waiting for their sons to reappear.
You won't be able to talk to anyone about your son's individual exam performance, but he has definitely done well enough to have passed the exams and to be seriously considered for a place.
There will be lots of staff around that you can ask general questions, though. We asked things like how many boys sat the exam, how many interviewed, etc, and they were very happy to answer these (as well as more school-life related questions, of course!)
Just make sure that your son sounds keen in the interview - I know that sounds obvious, but it definitely makes a difference! ask my friend, she'll tell you.
This thread takes me right back and makes me very glad that the 10+/11+ days are behind me. Good luck to everyone still waiting, and for interviews, and commiserations to those that have had no's 