It depends on the forum, I think. The forums where people flame each other are a waste of time in my opinion. It also depends how much time you spend on forums. There has to be balance between real life and internet time.
I think forums that provide support like this can be a real help. You can also find some really helpful advice, and just a friendly ear, to help you get along in the real world.
I've visited some forums which have been a real blessing like:
Knee Guru, when I was looking for advice on knee injuries, I also visit forums like cool running and the masters runners world, I visited a Haemochromatosis forum after I found out I suffered from this condition.