Depends what kind of pit your talking about, theres a few of varying intensity. You can have anything from bumping each other, shoving each other around, or pretty much a bare-knuckle brawl. Can't say I've been in many of the latter, but the first two are fun.
I most concerts I've been to have some sort of mosh pit. By far the funnest mosh pit was at a Reel Big Fish show; it was more of a big dance thing than a mosh pit though, it was great. The weirdest one was at a Dave Matthews band concert, I don't think anyone knew what they were doing.
Yes. Not very long. Vans Warped Tour 2005(?). There were two actually. Both were disbanded fairly quickly by the secuirity though. The first was mid day, but I can't remember who was playing. The second was at the end of the day just as the sun was setting at 8 pm. The last concert of the day. All the booths were closed and whatnot, so every single person was at the concert. This one lasted a lot longer because it was huge, and it took a while for the security to get to the middle of it.