I LOVE 'em!!!!!
They are a great tool for self-correction, as has been mentioned. I have seen beginning students make tremendous strides, just by subconscoiusly trying to make their motions in the mirror match those of an Instructor/Higher Gup student in front.
I am very greatful to have mirrors across the front wall for my sword class. They help with the single most important issue regarding technique in a sword art - making sure you are always leading with the blade edge. Mirrors help tremendously for making sure level cuts stay level and straight cuts stay straight.
My husband gave me my very own small workout room when he designed the addition (garage conversion) on our house. He included a higher ceiling, recessed ceiling lights, and a mirrored front wall, just so I could practice my sword work. What a guy!!