Comments like "nice", "great", "excellent" or simply ?
Personally I don't feel there is any problem with it as long as you give it impartially and not because the poster is your friend. This is definitely better than viewing, liking it and simply close it.
I know some are strongly against such comments and have to certain extent influence others not to post anything unless it is constructive. This is probably one reason behind the decline of critiques in the photo gallery section.