I've found everyone's product is a little different, it is understanding the offer and what you are getting out of the links they are building. Are they using handwritten content, what are the metrics of the sites they are posting on. How many outbound links are on the page or homepage. Are you using your exact desired anchor text or are branding. Guess it depends, I would make sure you thoroughly read any of the offers and ask the seller questions. Usually, I find if I ask questions, I learn a lot more than expected about the offering. Anyways, best of luck - Hambone