Hey @Kevin ask me again in six months! We're just holding our first event this month, so time will tell. Personally, I tend to favor groups that have a structured format, and that take steps to keep Long Talkers from monopolizing the conversation.
If you can keep the conversation reasonably focused then your best people will keep getting value, and keep attending. I'm planning to follow this Bootstrappers Breakfast format: http://www.bootstrappersbreakfast.com/first-timers/ and I thought I'd limit the group size to 12 or 15, at least to start.
I'm glad to hear you're putting this together. Let me know how it goes!