I think when used sparsely they're most useful. It's certainly preferable to putting on a film or something, when the students need a break. I think it's good to use them around exam times, or near the end of semesters. Also, as has been said earlier, if you make sure they speak in full, coherent, and grammatically correct sentences, then it has its valid use. I like to incorporate whiteboards so it tests the students on more than one area. Also, I've found that the best tool they are for me is to help teach something complicated or with depth in English. For example, the topics of Guy Fawkes, the Christmas Stories, Thanksgiving, etc, can be quite broad. It's a good way of testing, or cementing, what they've learnt in the bigger portion of the lesson beforehand.
Don't rely on them, but they certainly make a good addition.
If you pursue this career path, Korea will be one of the few places you can actually do this.
In the end, I hope they are useful to other teachers.
I'm spending ages on one at the moment...