1) primarily you’re getting the insight and “approval” of a blogger who already has a readership – and if you’re targeting your campaign well, you’ll be contextually relevant to their audience
2) blogs are more vertical than directories … and are more authoritative (this is an untested assumption-based claim)
3) a paid blog post is more likely than a directory submission to get linked to and passed around – whether it does or not will depend on the individual who’s getting paid
4) when one smart company gets its business together and provides paid-post distribution deals for related bloggers (like the techmeme sponsor post box) you’ll see your links spread far and wide
And please note that a MySpace Exec recently joined PayPerPost. To leave a company that’s jamming for a small start up is significant of confidence in a new model.
Will PayPerPost replace directory submissions in your paid link building strategies? They should certainly supplement them And watch for large PR firms to get in on the action too as they slowly grasp the power of blogs… and search engine marketing
See Search Engines Say OK to Pay-per-Post Services by Fishkin.