My bf always asks me if I'm going to my AARP meetings whenever I go to orchid related events

so don't expect too many young people to be there. Portland is a hipster kinda place though, so I could be wrong.
Experience differs from show to show so you really just have to experience one for yourself. Depending on who the vendors are will determine if there are any unusual orchids. I wouldn't say you're likely to find any rare ones though.
Vendors tend to bring what they know will sell. So, A LOT of them bring plants in bloom. This time of year it is the Den. nobiles coming into bloom, so expect a lot of those.
If you're looking for a score your best bet is to preorder. Some shows have seedling deals too, which I've found great value in. I don't feel like I'm getting a good deal buying any of the regular sized plants though. They're all pretty much the price they would be if you bought them online. Only difference is you save on shipping.
Best place I've found to get dirt cheap plants is at the monthly raffles my orchid society holds.
Have Fun at your first show!
