Here are some potential benefits of regular online forum participation:

Intellectual exchange
Learning new ideas and refining old ones
Enjoying community membership
Influencing the forum's evolution
Contributing to others
Making new friends and contacts
New business leads
Keeping up with current events
Learning about new opportunities

Here are some potential negative effects of excessive forum usage:

Reduced concentration and focus
Reduced productivity
Chronic procrastination
Increased pessimism and/or apathy
Being distracted by endless debates and idle gossip
Gradually substituting tribal group think for your own intelligence
Impaired social skills, neglected relationships, and a weakened social circle (a consequence of substituting online socialization for face-to-face conversations)
Reduced energy (forum participation is sedentary compared to more active social outlets)
Reduced self-esteem
Career and income may suffer (including loss of employment)
Forum addiction