We have been for the last few year and have enjoyed it, so will be going again this year.
As has been mentioned it was originally a Maserati Club event but has been opened up to any car "enthusiast" and so has a nice mix of cars and owners. Like most groups (dare I say, even here) there may be a few who feel there is a special "clique" but if I don't fit in so be it, I go for cars, food and for those not driving there is plenty to drink.
As for trousers, I normally wear Jeans but suspect if you wanted to wear bottoms to compliment a Burberry baseball cap I suspect you wouldn't be turned away. I believe it is restricted to those that have bought tickets, I don't think this is for any elitist reasons but purely so the pub knows how many to cater for.