I'm not sure what BNI stands for... I presume some sort of network group. I am a member of a Chamber of Commerce leads group that is VERY active about leads, and even tracks the amount of money people refer or receive from referrals. I managed to get into the most popular one of the groups. I don't personally attend that one... I dispatch my general manager to that one, although I did attend once.
The group that I am most heavily involved in is the Little Rock Executive's Association. It was very difficult to get into this group, but it has been EXTREMELY lucrative. We meet for breakfast every Tuesday and we have open houses every other Tuesday evening.
Attendance really is mandatory. If you miss too many meetings or open houses you get booted out, and there are many other businesses clamoring to get in at the first opening.
We have about 110 members, and they are probably the most influential people in the city/state. We have one person from each industry, and we get the creme of the crop. We have the top commercial builder in the state, the top home builder, the president of the biggest local bank, CEO of one of the largest hospitals, the owner of one of the top car dealerships, the owner of one of the hottest health clubs in town
... you get the picture.
As Chris mentioned with his experience, the first year I didn't make a ton of money off of it. Maybe a few thousand or so. Now I'm doing that every month with all the people from this group. My boot camp is a hot item. People either join it or send their kids/spouses to it.
The biggest payoff isn't just the individuals from the groups though. They all head major companies, and they give me the opportunity to come talk to their staff, and THAT is where you get your big payoff.
The thing you want to do no matter what is to get in front of as many people as you can. Even if they don't come to you right away, you are sowing seeds for future clients. You will be the first person they think of when they decide to do something about it.