HopeDream, I went to a three (paid for one that catered to the lower end of the market, free tickets to the mid-range and high end ones). I think going to an expo was useful early in the piece because it gave me a broad idea of prices and access to lots of vendors in the same room. The low-end one was pretty crowded, quite pushy, and not very personal. The high end one was divine (free canapes, capped numbers, fashion show, free alcohol) but I couldn't afford any of the vendors
My advice (if you're asking) would be to pick one that had vendors in your budget and enjoy as much as possible. Try not to get distracted by all of the "things" you "need" if you don't want them and concentrate on chatting to people you might want to use. I have to admit that I used recommendations from local forums when it came to actually choosing vendors, but the expo was part of the "wedding experience".
PS. I took my FI to the fancy-schmansy expo, my mum to the low-end one, and went alone to the third. Your company does affect your experience and I knew my FI would HATE the sales tactics.