I would call myself a long time lurker on HN and have recently started commenting on stories. There's no real reason why I started, I think I just finally felt like it.
That said, I have been reading the comments for some time and I think the quality has changed when it comes to items that crack the front page. Beyond the first 30-40 items I think the discussion has remained far more useful than not. That's not to say that discussions worsen as stories move up, I think the volume tends to increase which invariably brings less helpful comments. This is not hard and fast, just my anecdotal observation.