I've always been vaguely aware of CSS Lint, imagining it to be a useful tool with a good reputation, although I've never used it before today. I gave it a go earlier (following a link in this article on a A List Apart) and I was massively unimpressed with what it suggested needed 'fixing' in my code. I was going to write a post about why, but happily someone's already done it:
CSS Lint is harmful
I don't really have anything to add. He covered everything that bothered me about it, and then some. I'm also very pleased that it's the second result when googling 'CSS Lint.'