Rather large time commitment. You will constantly uncover leads for your fellow members and that takes a good deal of effort/energy. Although they don't have a "requirement" for leads like LaTip, you will instantly stagnate if you don't develop a personal plan for generating leads for those members. You might have a few personal referral conflicts due to established relationships with professionals outside your group. Yep, some groups are duds, but that's because of the individuals that make up the group. Are BNI members the cream of the business community?
1st year is $400. Have a boat load of energy and set aside the time required to generate leads. If you don't do this you will be just like a lot of other still born members who simple show up to the meetings with fake smiles and no commitment to generate leads for YOU. I agree that picking a "good" group is important. The last element I will leave you with is your ability to communicate your needs to your group. Every week is your opportunity to teach your group how to sell your business or at the least how to be trained bird dogs to look for your favorite customers and your everyday bread and butter customers. It's still 90% you and your abilities to communicate.