Testing is one of the "Runner blogs" as I shall call them for now. Our protagonist (Shaun) "met" Slendy when he was 17. He knew about him from before, and when he finally couldn't deny Slender Man being real, he decided to run away. First, to not let Slender Man ruin and eventually end his life, and second, to protect his girlfriend and family.
At the beginning, he mostly describes "experiments" he made. Like, testing if Slendy prefers night or day, lit or dark rooms and the like. Of course many of these experiments require exposition to Slendy, which makes this story pretty exciting. Shaun keeps interacting with the protagonists of other blogs (for example, he regularily talks to M from "The Tutorial" http://getuphigh.blogspot.com/), so reading the comments is very important on this one.
Around September, Shaun gets in contact with Robert Sage (http://fighthimuntiltheend.blogspot.com/) and Jay (http://anomalousdata.blogspot.com/). I had a look at both blogs; Jay seems to start with a quite scientific perspective on Slendy, while Robert is more on the 'mythical' side. Shaun starts to interact with both of them, soon adapting Roberts idea of there being different "roles" in the Slenderverse for the people who have to deal with Slendy.
This blog is definitely well-written, and Shaun as the main character is very believable and fleshed out. The only thing that bugs me (but thats only my personal taste), is the constant 'cross-blogging'. I don't want to be forced to read 4 different blogs, to be able to follow the storyline of one of them. And I tried reading Testing without looking at Roberts and Jays blog. I was absolutely confused. If you're the type for connected blogs, this one will surely entertain you, even if Shaun recently pulls back from the other blogs, and is more focused on his own thing.
Very interesting sidestory, believable character, neat descriptions of Slender Man's influence on a person and an intriguing writing style.