I'm responding to this a tad late, but whatever. Haha.
I can't speak for all meet-and-greets, especially since I've only been to one for Train, but I can certainly compare with other M&Gs I've been to!
The M&G last year in The Woodlands was before the show. I, too, have been to M&Gs that are done in big groups--where your photo is taken with a bunch of folks you've never met before--and this wasn't like that at all. In The Woodlands, if you bought one pass, you met them individually. Two or three passes, and you met them with your friend(s). If you bought four or more passes, you had the option of taking one group photo (with only your friends/family) or splitting into pairs and taking two photos like that. Essentially, there will not be strangers in your photo. ;)
As for time, there was a fairly long line of people at the M&G in The Woodlands, so no, you didn't really get to stop and have a long conversation with them. Alas, it was a meet-and-greet, not a meet-and-discuss-national-politics-or-whatever-it-is-people-normally-discuss-for-extended-periods-of-time, so conversation was limited to a minute or two. ;) You pretty much handed them something to sign (I had them sign my M&G badge), chatted for a bit, had your photo taken, and headed off to the show. It wasn't as cattle-herd-y as other M&Gs I've been to, but it was indeed brief, as these things tend to be.
Hope this helps!