Yea I wouldn't be using it for an accurate bf% calculation but rather to see how the skinfolds go down. But like you mentioned it would seem like even that can be altered by water and the like. Similar to waistline measurements :\
Mirror can be useful, but its hard to judge over a period of time and lighting, distance, depletion, etc. obviously play a huge role in how good you look and unless one is very lean I think weekly progress can be hard to judge with pics/mirror.