Open Days are useful for some things, for example compiling a short list of schools which you can revisit properly later. I don't think you can make a full and final decision about whether a school is the right one based on Open Days alone.
It is difficult to judge a school in isolation, so definitely worth looking at some others even if you are pretty sure about your first choice. For one of the selectives you might want to have a back up option anyway, and so will need to find two schools that you like.
When we were looking around we felt that the best way of getting a feel for a place was by talking to current pupils. It doesn't suprise me that the WC boys were steering well clear of Open Day chaos this weekend, but on a private visit you will always find boys to talk to in a more relaxed atmosphere.
Also, Open Day parents are not necessarily representative of actual parents, so don't let them put you off either
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So long as you like the aims and ethos of a school, if there is a clash it's more important to like Housemasters, Matrons and teachers rather than the Head in my opinion. It's my impression that in these big schools the Head is more of a political figure, responsible for the overall public image of the school and acting as an interface between the school and the outside world, rather than being involved in the day to day care of the pupils, as they would at a prep school.
A final little thing for those just starting out, I really wouldn't at this stage get too hung up on details like dining arrangements. I have several friends whose DCs had quite specific requirements, which were then completely forgotten about when they found a school they liked.
Like that property programme with Phil and Kirstie when couples have one or two points that they absolutely won't compromise on, but which are usually the first things to be abandoned when they find a place they like!