I´m working as a webmaster for a site that runs a forum with now about 9000 members containing about 90.000 posts. It can be quite interesting to run a forum, especially if you´re interested in the topic the forum is about (to be honest: you HAVE to be interested in the topic, otherwise you won´t be able to deal with the great amount of work, that comes when running a forum

). It can also drive a lot of traffic to your website and you can earn money by using Google Adsense or similar programs.
But as i mentioned, a forum means a lot of work:
- a dead forum doesn´t look good. So you have to do a lot of promotion for the forum (online and offline)
- you have to deal with spam
- updates of the forum software have to be installed immediately when the were released (to prevent security risks)
- you have to check regularly what´s going on in your forum. Sometimes I had even "battles" between forum members - and you´re also responsible for the content of the forum (even when it´s created by members)!
- there are many forums all over the internet about nearly every topic you can think of - so it´s quite hard to find your niche to make users join your forum and none of the 23.000 other forums about this topic.
So all in all: you have to be 1.000 % positive about your forum topic, otherwise it won´t work.