I have been to a few BNI's over the years as a guest until I found a group that was right for me. It's a good idea to go as a guest so you can get a feel for the group, it took me 3 go's before I found a group that was really into referrals and not just turning up for a chat.
Different industries get different results in regards to sales and often it can take 6 to 12 months to get a qualified referal as people need time to build report and trust you what you have to offer.
In my opinion this is too long a wait only because so much money is involved; the sign up fee, then the weekly fee and if you miss more than 3 meetings they can kick you out and no, you don't get your money back.
I am totally into social media (hence my business
www.socialbuzzonline.com.au) as you have 1000's of people you can communicate with every day. This makes much more sense to me, than attending the same meeting every week with the same 30 people.
Good luck with your decision, don't forget to look for non BNI groups as there are many networking groups around including free ones.