I've had some experiences with Ouija Boards and it's really whatever you want to believe. I've had some creepy, yet slightly coincidental things happen.
In my opinion, I think it's caused by the "Ideomotor Effect", when you move something unconsciously. On a slick board with 2+ people holding onto a smooth planchette with the "excitement/fear" of doing this, you may begin to move it slightly and spell out things that you want the spirit to say.
The only real way to test it is to have you and your other friend do it blindfolded and have a 3rd person to record the movements. If you get something, in this case, you've made a believer out of me (unless you've memorized board movements, of course), but unusually blindfolded, you'll get a mixed up alphabet. Thus, why I think the ideomotor effect is the cause.
Hasbro makes these things as toys for $20. I don't think talking to spirits is in Hasbro's business plan.
They work about as well as a fortune cookie. You give it the power by looking for something related to what it said: "OMG my fortune cookie said 'you will come into some wealth soon' and as I was digging in my couch cussions, I found a nickel! Amazing!"
