I do a weekly breakfast meeting – i've been a member for about 6 years and it's more than paid its way every year, so that's OK. It's not a short term thing though, you get people who show up and expect to be minting it after a few weeks and that just doesn't happen – if you reccomend someone who turns out to be a turkey then that reflects badly on you, so there's a fair bit of 'testing the water' before anything really emerges. I've had intros to jobs that i'd never have got any other way, so if you're prepared to do the leg work then it can pay. Just turning up for a lunch now and again won't do a damn thing.
The other big thing is to see who else is there – if you're a financial adviser and you go to a group where there's a mortgage broker and a solicitor then things could work well – if there's no obvious connection with anyone else then it'll be that much harder to make it work.
So take your time, visit a bunch of them and decide what feels right for you and your business, then commit to attending those meetings. Attending the right one or two good networking groups will pay off better than scattering yourself too thinly.