I have participated on forums for a long time. I mostly frequent forums that have to do with beliefs and politics.
For the most part they are enjoyable. I like being able to share my views and show off some of my intellectual side in debate. I love discussing beliefs, interesting ideas, etc. It's definitely a plus to get into topics and details that you normally wouldn't get into in real life. But, I also think there is a downside to being on debate forums too much. At least according to some people who I've known to quit forums (or take a break from it) after a while, some of the negativity on forums can carry over into real life. The negativity I'm referring to is becoming too defensive from having to always defend your views against skeptics and others. But then again, this all depends on the environment of the forum and the types of debate.