I'd rather get them on the not so good ones, especially when I thought I had a good one and it is bombing in the 4's. Those are the hard ones to comment on, though.
I'd rather not get any comments at all, it is a major bother having to tick "this comment was helpful" all the time. ;)
But yes both are good, both positive and negative comments, I want to know what people are thinking when they look at my work. It drives me mad to see so many look and say nothing.
If we know which of our images are "not so good", we should remove them. (I wish we could do this with past challenge entries, too).
If all of us would publish only our best shots, learning, IMO, should be more effective and enjoyable.
There's always a Workshop folder for experiments and tech examples.
As to comments, let the writer decide what he wants to comment on.
Its a bit difficult for me to comment 'Your photo is really bad, I cant provide any tips since there is nothing that can be done to make this any better'
I like constructive criticism. Sometimes I take a photo that I absolutely LOVE and then I enter it in a challenge and it totally bombs and gets almost no comments so I have no idea what's wrong with it. That doesn't help me improve, it just frustrates me. I thought my negative challenge entry was really cool looking and it totally bombed and it didn't get a single comment the entire week. I was really bummed about that because I didn't know what was wrong with it. Someone finally commented after the challenge was over that it was too dark, which I appreciated hearing because I have a bright laptop screen and I don't always realize how much darker it's going to look on other peoples screens. At least now I know what was wrong with it, or at least what one person thought was wrong with it.
It's always great to get the positive comments about how great your entry is, but I find the negative ones more helpful if they suggest something that I could have done better. The reason I come here is to learn.