I've been to most of them, and would agree that if possible go to all the days, but there will be plenty to keep you entertained on any individual one. The Friday is practice, but I think you'd see all the entrants run.
Over the road is good too, but if you've only one day, I'd leave it till later as there is plenty inside the circuit / pits.
Do walk right round the track though - the pits / chicane is mobbed with poor viewing, but all round the back is MUCH quieter, and there are big screens to follow the leader board etc.
not sure what us H&S bods have done to drag it down though.....

- the event is certainly huge now, and many folk are not really there for the cars / racing, so there is a lot of stewarding required. Mind you, I've never seen any of the drunken violence that Goodwood was in the press for at the horse racing the other week, but H&S'll probably be to blame for that too!
Al