I think your best bet (based on advice from this site and my own experience) is to find the community most relevant to your product and make sure everyone in that community knows you're launching.
Reddit is a great site to hang out on, but most of the people on there - at least from the comments - are college kids with no world or business experience.
Grain of salt? There isn't enough Sodium on this planet I can read Reddit with.
Comments are garbage most of the time unless you find some super niche undeveloped subreddit unplagued by the masses, only retaining passionate users.
What happens is a community starts with people passionate about the topic of the community, and then the focus shifts from content to community preservation once the masses leak in and it devolves from there.
Because it's unmoderated for the most part (subreddit moderators don't count, most are bad at what they do, source: most subreddit content is bad), communities quickly devolve into the lowest common denominator of discussion, memes, bad unfunny overused jokes, and puns (which I really don't mind actually).
Sorry, I have a severe distaste for Reddit once I realized how "dumb" it actually is. I realized this when I saw articles posted to Reddit from HN that hit r/all and how different the comments were. Reddit for the most part seems fueled by hormones, HN is level headed.
I think your best bet (based on advice from this site and my own experience) is to find the community most relevant to your product and make sure everyone in that community knows you're launching.