I started paid forum posting not because I need the money, my day job as a computer programmer pays me enough (although don't tell my employers that :-)) but I was bored. I like posting on forums however I rarely have any motivation to.
I thought that getting paid for it would be a good way to keep me interested. Due to the nature of paid posting jobs you are never on one forum too long so it gave me a varied set of forums to post on.
The idea of paid forum posting is easy from a posters point of view. Get a job, the company I was working for you did this by picking your own jobs from their control panel. Once you were assigned to a job you signed up to the forum and posted away. The jobs tend to be structured by the number of new threads and number of replies you post.
I was picky about what kind of jobs I did but it quickly became apparent that most of the posters weren't. The quality of those posting from the same company was very variable from those who could barely string an English sentence together to those who were just in it for the money.
Quite a few of my fellow paid posters just weren't worth the money they were being paid, most of them ticked most of the problems I found with posters below on a regular basis.
Certainly there was, however most of the good posters would get drowned out by the dross. On the rare occasion I had a posting job with mostly good posters it was a complete pleasure. It wasn't always the posters knew their subject matter, but they did know how to have a useful conversation on any topic.
The particular company I was working for would give you a list of which posters were paid posters. In the majority of my jobs the the people on that list would be the only posters. I would imagine that the person that created the forum would know their subject and would be interested in promoting their own forum. I was shocked, these people are paying money to have their forum filled and they don't seem to be keeping an eye on what's going on.