I was a BNI member for 4 years. My primary goal was to expand my contacts for selling my employee benefit. It didn`t help me much, although I covered my expenses by selling individual coverages.
Some have mentioned that realtors/mortage companies/title companies do well. In reality if you have any niche group that works together it will benefit all of them, For exanple wedding photographer/wedding consultant/DJ service/florist..., or the construction industry with painter/handyman/contractor/plumber/electrician/banker(home improvement loans).
There can be a problem with some people playing business, and with some places that are extremely difficult to make referrals for. But BNI`s code of ethics helps to insure that your referrals are treated well.
The group also helps you learn how to do a 60 second pitch for your business every week. You can learn a lot from the structure of the meetings and from the interaction with other business people.
But that being said, BNI is not a good fit for every type of business. Be sure to visit twice before making a decision if you want to join.