My problem isn't with the whole "speaking in tongues" thing directly - even Orthodox saints have spoken in tongues, not as a way to evangelize to those who speak another language, but as a divine language that no one else can interpret. My problem is that their "speaking in tongues" is most likely of a demonic nature, both because studies have shown that they're just doing random blabbling based on their own linguistic expectations, and because the apostle Paul strongly warned against speaking in tongues in the open like that, specifically because, if they really are speaking in tongues, then it must only be between them and God if there's no one to interpret, and they'll drive away onlookers otherwise. "Speaking in tongues" during a liturgy when it's not a real language that somebody else can understand is probably demonic if you're not doing your best to avoid doing it in front of others like that.
Incidentally, I went to a Pentecostal service yesterday afternoon, and I was deeply disappointed. Besides the unrestrained dancing and "speaking in tongues," which shows absolutely no reverence, there was very little actual Bible in their service as well. OK, singing about how much you love Jesus and how much you want His Holy Spirit for over half of the service is cute, but when I'm going to worship God, I expect to hear the beautiful sound of the Scriptures ring in my ears, and there was very, very, very little of it. It made me seriously miss the Divine Liturgy, if only to actually hear the Scriptures.