I looked into this question in a guest post that I wrote for the Leukemia and Lymphoma Society's blog.
I wrote about how I started my blog (with an April Fool's joke), how PJ helped me start navigating the system, and how I discovered the positives: connecting with kindred spirits, finding support and providing it, easing isolation, telling funny or (I hope) meaningful stories, and framing my thoughts in a way that often helped me find the silver lining.
I also wrote about the negatives: the sadness in forming connections with people who don't make it or coming upon blogs carried on by relatives of those who died, reading of other patients' medical problems and then worrying that I might develop those problems too, feeling neglected when I don't get enough comments, and spending too much time on the computer.
Another "con" that I didn't write about is the tendency to assume all of my friends and family want to read it and comment. Some do, which is fun, but others don't. We're an older group that didn't grow up with all this computer stuff; not too long ago, I didn't even know what a blog was, and some of my friends don't want to ever know, let alone participate in one. I tend to get a little obnoxious when I really want someone to read it, or when I'm just too tired (or sick of hearing myself talk) to explain the results of my latest clinic visit. Sometimes I say, "The details are on the blog," but really they want to hear it from me.
Sometimes I blather on.