I was in BNI for almost a year and it was useful. It cost £500 to join then £8 a week for the breakfast - I got work worth 4 times the joining fee in the first month. After that I got a steady stream of small jobs.
It does require a fair amount of commitment and as dp alluded to above it is quite americanised.
There are lots of other networking groups out there including Chamber of Commerce etc some dont have any joining fees - you just pay per meeting.
Most allow you to go to a meeting or two for free before you decide to join.
If you want to get more business then its worth a try in my opinion.