Talk to as many vendors as you can and get their contact info. Even if you already have a specific vendor (catering, florist, photography, etc), you never know what will happen and if you will need a new one due to some unforeseen circumstances.
Also, enjoy yourself! It can be slightly intimidating and overwhelming, but just have fun (:
My best advice is go in with a goal.. we went to one a few weeks after getting engaged and we went with the goal of getting venue ideas. We collected every brochure from the venues there, spread them out on the dining room and picked a handful to go visit. We booked our venue officially within two weeks of the expo. Of course get the info of other vendors you may need to look at later, but it can be very overwhelming. I’d just have a clear picture in mind what you need at that moment and really focus on that
If you haven't yet, create a wedding specific email and ONLY give that out. You will be flooded with spam and it is so much better to have it in one place and not clogging up your regular inbox. (This will also be super helpful throughout planning because all email correspondence with be in one place, and you can share access to that account with others if you want/need to.) Also, be wary of "special discounts if you sign today"; that pretty much guarantees you won't have time to do homework and/or check references. Finally, realize that some of the people you talk with (especially photographers, DJs, etc.) might actually be brokers for independent contractors. If that's the case you'll have a lot less control over who you'll actually be working with for your wedding -- you'll be guaranteed a photographer, but may not get much choice about who. Good luck! We found it to be a fun thing to do right after daughter got engaged, but not really productive. (Keep in mind, that most awesome vendors are booked for weddings on a Saturday or Sunday, so they don't typically attend Expos.... Many who are there are just starting out or are in the process of building their business. That's not necessarily a bad thing, depending on your budget and what you're looking for. But most really great vendors get plenty of business through word-of-mouth, rather than participating in Expos. Just something to consider.) Have fun!
Also, realize that virtually everyone who attends will get an email/phone call saying "they've won" something -- however, many of those will be scams to get you to attend a timeshare-type presentation. Just be cautious!
I went to one and didn't find it that useful to be honest. However it was fun because I went with a bridesmaid and I guess if this is your first exposure to anything wedding related then it's a good way to get ideas.
We have been to a few! Some of the local ones put on by actual wedding venues were found to be the most helpful as far as vendors and ideas. They used a lot of local businesses as well as very POPULAR vendors for weddings.
We have a huge bridal bazaar in San Diego that was honestly not worth our money! We kinda went for the free gift bag, and free stuff lol but there were so many vacation sales pitches, mainstream restaurants for catering, and it was honestly mostly just mainstream vendors and we weren't into it.