Helped someone with some paid posting once. Picking quality blogs is, as Bogart says, the tough part. As he described it, it's only worthwhile if the posts are, so you're talking about manual traffic, not google value. A quality poster on a gun forum or a gun blog with a gun product in their footer might see traffic. A guy in this place or my blog pushing replica handgbags would get none, and he'd get tossed quickly so any posts he got up would be gone or missing his link.
I see value in having a link when you post if you do it in the right place, but that'd likely only be something one did as a byproduct of talking on blogs where he'd probably post anyway... not an seo campaign.
I turned off comments for most of my blogs because 99% of the comments were spam. I got hit so hard by the Chinese comment spammers that it overwhelmed Akismet and shut it down.
I don't think a comment does any good for driving real traffic unless the focus of the blog page is directly related to your site. If it is not, then any traffic is non-targeted and is probably useless.