They can be useful or they can be a waste of time. If you are going just to collect information, you can do that via mail request. If you have specific questions that you can't or haven't be able to get answered, it is a good thing to take advantage of.
I went to the one in Atlanta last year and I was able to get some good information. I got good vibes from some schools, and got turned off by others (Vanderbilt). You can ask them questions directly without having to wait for return emails/phone calls. It just depends on what your needs are. You may also learn about a school you didn't even think to apply to, which happend to me also.
I am going to the one in Atlanta tomorrow, and this year I have more specific questions.