Hi Phil,
Very valid questions. I went to a few Nationals in the 80's and then was able to make Cleveland in 07 and Chicago in 08.
As many posted above, it is a great opportunity to meet fellow collectors, see awesome stuff in person and just enjoy the atmosphere.
As for my experience, I loved working on upgrading my 50's and 60's sets and prices were resonable and selection was over the top with multiples to choose from of most cards which makes it more fun for a centering freak like me. I really enjoy spending hours going through boxes and pages of cards looking for cards that appeal to me. But that's me
I did find some pre-war cards that were priced reasonable but I think you have to really look to find these. Picked up a couple of T-204 Ramly's in Chicago that I was happy about. Most Pre war is priced above ebay auction's but heck, there has not been much on ebay lately.
I think you have to go if you have a passion for collecting and just take it all in and have fun. I will not be able to make Baltimore this year but will be back in action in Chicago in 2011. Hope to see you there.