Constructive criticism, as long as it's given in a polite way, is always helpful. From a mentor's perspective, if it's a review of a student of mine, I don't publish it until the information is correct. But if it's a review that's published on the site without a mentor with blatantly incorrect information, I will usually leave it nicely in the comments.

This is pretty much my thoughts exactly. As an editor, it depends. If it's something that could cause some serious problems, I sometimes leave it as a comment on the unpublished review. Other times I'll leave constructive criticism on the review itself.

Exactly. I don't take students, nor have I applied to edit reviews, (just haven't gotten around to it. I'm not even sure if I want to do it.) but I do leave kind constructive criticism and feel like you do when improper or dangerous information is posted on a review. I hope people take the criticism in the way it was intended: to help them write better reviews and to make sure the data given out is correct.

The way I look at it, I spend a lot of time and effort on my reviews and have since my first one. I expect no less from anyone else. Also, if the data is dangerous (the oft used "you can use this in your butt" and it's obvious you shouldn't) I will say something, hopefully the reviewer takes it gracefully.

If it's an ethical problem (like a review taken word for word from someone elses' review) I contact the Administrators. This doesn't happen often, but does on occasion.