Darren Rowse calls Buy Blog Comments "one of the worst business ideas [he has] heard for a long time," and I am obliged to agree. This is not only a monumentally poor idea, but one that is potentially dangerous for the blogosphere as a whole.
Comment spam is a big problem, and it comes in a few different flavors. There's the type that looks like spam (just a lot of links viagra or porn) -- this type is usually caught by anti-spam plugins for blogging engines, such as Akismet. There's the spam that consists of a generally plauditory generic comment followed by link -- though slightly more readable, this is still clearly spam and easily removed. Then there's the type the Buy Blog Comments is purportedly peddling. These comments are generally on topic, but have the underlying purpose of self promotion and not of improving the conversation.
Buy Blog Comments charges $.20 per comment for what they say are "quality blog comments." To write truly quality blog comments that won't be flagged by site owners as spam like the site promises one would have to find related blog posts, read them, and compose thoughtful, on-topic replies that subtly weave in a marketing message with a link that is worthwhile to readers. If we can assume that takes at least 10 minutes per comment, then this site is paying its writers probably less than a $1/hour. That makes me skeptical that the site could deliver on its promise of comments that don't look like spam. But I'll let the site speak for itself and you can judge the "quality:"
We dont use people who cant even speak english. It is important to have well written blog comments so that they wont get deleted by the blogger. All of our trained staff are currently from the USA and Canada and speak english very well.
The site is run by Jon Waraas, a 20 year old entrepreneur who in the comments of Darren Rowse's site admits that he's "just not very ethical." Waraas adds to the shadiness of an already suspicious endeavor by quoting himself on the Buy Blog Comments web site in a manner that looks like a customer testimonial. The site itself says that it specializes in selling to "blackhatters" (a term used to describe people who use search engine optimization techniques that are frowned upon by the search engines or outright against their terms of service), which is a good way to attract negative attention from Google.
As Search Engine Journal notes, there is value in comment links, but paying someone to spam blogs (or doing it yourself) is not the way to go about building backlinks.