Here is a website to clear some doubts on speaking in tongues and all tt. just 2 paragraphs taken frm there :

Then ask yourself the question, could I, through this means be deceived? Could this be dangerous? The answer to that question has to be yes. A man whom I knew and respected greatly, now with the Lord, George Gardner, who was pastor up in Grand Rapids, who wrote a very excellent book on this subject was a former tongue speaker who left the Pentecostal movement, and he poignantly described the danger of surrendering one's mind and abandoning control of one's self for the sake of the euphoria of a tongues experience.

He said it's a very dangerous thing and this is what he wrote in his own words, "The enemy of the soul is ever ready to take advantage of an out of control situation, and thousands of Christians can testify with regret to the end results. Such experiences not only give Satan an opening he is quick to exploit, they can be physiologically damaging to the individual. Charismatic writers are constantly warning tongue speakers that they will suffer a letdown, this is ascribed to the devil and the reader is urged to get refilled as soon as possible. So the seeker for experience goes back through the ritual again and again, but begins to discover something. Ecstatic experience, like drug addiction, requires larger and larger doses to satisfy.

Taken from:
http://www.gty.org/Resources/Transcripts/90-61