I only went to one (the one in Columbus, OH where Randy beat Sylvia). I got there the day of the event, but I've heard that it's usually possible to meet fighters in the days leading up to them if that's what you want to do.
I would definitley get there on time. I was used to small shows and boxing events where the start time was almost always delayed. The place was packed and they started the fights right on time--I missed a bit of the first one getting a beer.
The fighters who were attending the events were definitely accessible. You could tell some guys (Shonie Carter for example) really loved to talk to everyone and other guys were more reserved. Your biggest problem in talking to them will be pushing through the swarm of other fans that follow them everywhere.
I guess every arena is going to be different, but I had ground floor seats and still ended up looking at the monitors most of the time. In other words, your view from anywhere but right against the octagon doesn't really matter because the camera angles for TV are almost always better.
Like smaller shows I've gone to, the people next to me talked my ear off before/between the first fights about how knowledgable they were but didn't know much. If you let on that you know something about the sport they'll ask you about what happened in every round. If you want to be left alone just nod your head and go along with whatever they say.
The UFC runs a great event. It really comes across as a big deal and its very precisely run.
Have fun, bro. Don't boo when they go to the ground