Personally, I'm a proponent of security vestibules. Admittedly, I don't think like a non-banker, but I think the message conveyed to customers is "We're here to serve you, and we're not going to let some nut with ill intentions keep us from that goal."
We installed a security vestibule in one branch in response to a rather troubling robbery. After the robbery, our customers were concerned for the safety of our staff, as well as themselves, and were very glad to see that we took steps to address their concerns.
And unless you're customer lines are too long to fit in your lobby, forcing one-at-a-time entry is simply a non-issue. And it's not difficult to accomodate adults accompanied by their children, physically handicapped customers, etc.