Noah Lorang reporting on 37signals' stockpile of data:
During that period, there were 24,826 first time visitors to any of our sites who we could identify as having first gotten to us via Hacker News (in all, we received more like 105,000 unique visitors from Hacker News, but many of those were repeat visitors). 97 of those visitors signed up, with more than 85% of them electing the free plan. This conversion rate pales compared to our average conversion rate, particularly for non-search-engine traffic.
When all is said and done, what’s our likely financial outcome from Hacker News visitors for those 25 posts? About $300 total per month.
In short, unless your audience is hackers and other assorted startup geeks, if your marketing plan consists of "getting on Hacker News" you're probably screwed.
Some might object that 37signals is a special case, because everyone on HN knows about them already anyway, but this jives with my experience, that HN sends plenty of traffic, and they'll have quite a few insightful comments, and a few of them might even be operating in your industry, but the net benefit of getting on HN is advice, not traffic.