I don't think some of those are even private blog networks, or atleast ones you should be using.
Here is my personal advice for joining a private blog network.
1. Make sure your link is on the PR of the page they say it's going to be. Some networks will say PR6 links! You even see this crap in BST forum. Then your link is on a subpage that gets pretty much no juice from that PR6 page. So you are basically paying for a link with very little value. You might as well submit an article or setup one profile linke instead of paying for a service.
2. Make sure the outbound links are capped, preferably around 100 or less. Think about it. If there is no cap, they could have 1000s of other links on that page, which is no different than the flooded blog comments you see. Why do they have very little value now? Because there's 1000+ other outgoing links on that page.
In my opinion, those are the two biggest things to avoid when joining a network. When I say my opinion, it's my opinion, from someone who always goes for the highest quality links possible. I don't believe in cheaping out when it comes to SEO. I think cheap services/links are short term gains (if that), while quality is for the short and long term.
Look at Panda update, it minimized the value of a lot of different techniques, a lot of time and money wasted by people.
