A pianist
While I use a mirror for almost all of my practise, I don't have room on the left side of my acoustic piano and always keep it on the right. The other day I played on a friend's piano and they had a mirror on the left. When glancing over, I realised that the arch on my right hand is still absolutely terrible in various places that I had never been aware of. Since then I've been practising a lot on my digital with the mirror on the left and noticed a very big improvement. I think the fact that my right hand quite so much worse than I expected probably shows just how much the feedback from the mirror must have been helping my left hand. I'd assumed that my right hand was probably working in much the same way as what I saw my left doing- but it's collapsing all over the place! I think it's a very good idea to line up a mirror- but to keep changing the side on which you keep it!