The HN guidelines say "Please don't use HN primarily for promotion. It's ok to post your own stuff occasionally, but the primary use of the site should be for curiosity." Moreover, such posts are likely to be downvoted and flagged into oblivion by HN's readership for being spam.
Please don't. I didn't come here to see advertisements and seeing them is something that makes me not want to use a website.
I like HN. Please don't ruin the community with ads.
HN permits some level of advertising in the "show HN" class. Thats about enough for me.
It depends. If people are asking "How do I do X?" and "What do you think about products in category Y?" I am certainly go to say "try this out".