Dangerous enough to get you a penalty. Amateur webmasters like to think of themselves as experts in SEO, so instead of hiring an expert, they prefer to save the money and throw some backlinks that they find very cheap online from third-world countries mostly. Bookmarks, forum profiles, bloc comments, web 2.0 sites, link directories, article marketing, guest posting, any gig goes as long as the price is right, and if it doesn't work, no worries mate. Well, I see the practice going on for years, and it has serious consequences. Despite the information that Google discloses, the warnings, the increasing number of penalties, the AI, the innovations that create new highways in search, despite our efforts to change the mindset, people still get hit by penalties every day. There are fantastic gigs out there, but when it comes to backlinks or SEO, gigs with backlinks blasts, comment blasts, web 2.0 sites, forum profiles, will get you in trouble.