Limmer gave some great advice! And I agree with her, if you have most things planned, don't bother.
I went to two, both early in my planning. I found my photographer at the first one. Other than that, I was doing the rest of the stuff (food, flowers, decorating) myself. The most helpful part for me was to wander around and look at all the decorations. We got some great ideas on how to decorate this way.
I also won some freebies by going. I got $50 off my photographer through a drawing (not a lot but every bit helps!). I won invitations from a tux shop (I took the credit instead of the invitations they offered and was able to get beautiful invites for cheap!). They drew names from the forms that were submitted. I won a $50 GC for Gordon Food (a bulk food place). And I won $25 GC for a party store, where I got our plates and napkins.
If you do go, get pre-printed address labels w/your name, phone number, addy and wedding date on them!! I printed them off my computer with labels from the office stores. There are *so* many things to sign up for and it takes forever to fill out all the forms. I was skipping it by the end of the second one because I was so over filling it out!