There are benefits and disadvantages to training in a room with mirrors. As an instructor, it is a great tool to observe the students. As a student, I can use the mirror to observe and refine my movement. The biggest disadvantage I found was the reliance of the mirror as a visual cue in kata.
I can remember two instances where "the mirror as a visual cue" caused me trouble. The first instance was during a tournament. I was in a black belt kata division and I was demonstrating Chinto. This kata is known for its quick directional changes. I was practicing in the dojo for weeks. I stepped in the ring and began the kata. I completely lost orientation and did a kata that sort of looked like Chinto. The second example was during formal testing. Testing was held in a restaurant banquet hall. The hall was lavishly decorated with bold wallpaper. Ever wall looked exactly the same.
I learned a few lessons...
Kata needs to come from the inside. It is important to know your kata well enough to be able to demonstrate in any venue. It doesn't matter what the people are doing next to you or what noise is in the background.
Practice kata in different directions. We regularly practice kata away from the mirrors. After we know a kata pattern, we try to break the visual cues by facing multiple directions. We even practice kata blindfolded.
Tuesday Tip: Change Direction...practice your kata in many different directions.