In my experience, a lot depends on the people involved in the seance.  If everyone has a clear intention of what they are trying to accomplish, and that they have good intentions, then I don't see anything wrong with them. 

It also helps to have someone 'lead' the seance who has had some experience in a spirit circle.  Not necessarily a full fledged medium, just someone who has an idea of what to expect.

If you get a group together who's sole intention is to try and communicate with someone like John Wayne Gacy or Jack the Ripper and they are doing it because it sounds like a fun and scary thing to do, they will likely attract a very nasty person who may not be interested in leaving when the seance is over.  However, if everyone in the circle is intending on communicating with spirit for the purposes of healing and compassionate communication, things tend to go much smoother.

Actually, same goes for any kind of spirit board or any other tools.  Intention is everything!  And don't forget to ask for (insert personal supreme beings name here) to protect you.

Wow, that turned out to be a much longer answer than I expected!