I have heard a lot of things (good and bad) about backlink kingdom (backlinkkingdom.com). I have always stayed away from such sites who lets you spin the content and submit it to unique c-class IP address. Especially after the Google panda update I am bit skeptic to even try out such sites. To be frank I have never tried out such 'easy to follow' method for one or another reason. 1) Number of paragraphs remains same in spun content 2) Position of link is identical (say in 2nd para or 3rd..it depends on you where you put it) 3) Not enough text variation in surrounding text 4) Anchor text is not spun. It uses %Link% (not sure if they allow to spin them) For me point 1) and 2) are really important. There are also many ways to find out spun article (unless you've manually re-write it). Have you tried such network? I would like to know your experience in using such sites or spun content.
I guess you need to do a quick google search for that. I came across a good one some days ago with this search string on google "free backlinks". Hope it will help somehow!
Depends on how you use it. They can be powerful if you use it to build complementary links. I use a similar service (see link in my sig), and it's working great for me - the key is building unique descriptions for each submission, meaning that the spun titles and text are not PLR or ripped from my site, but created for promotional purposes. Of the 25 links I do in a blast, about 30% get picked up automatically - and about 40% I submit to directories. That means in total I get about 70% of my mass links indexed. The remaining 30% I just submit later on or get Google to find them naturally. It works very nicely.