We attended our first one last summer in Louisville. We were on a parking lot in 90+ degree heat/humidity & it was HOT! We had a good time though. Many manufacturers were there with rigs on display, lots of vendors with stuff to buy, dozens of great seminars & fun entertainment (for the most part). I was disappointed in the food vendors they had, but we found several good local places to eat when we wanted to eat out.
If you've never been to one, I'd say it's worth it to see what it's all about. You can see pictures of last summer's The (Affinity Group) Rally
posted in my photo albums.