The only rap concert I've been to was the surprise opening show for the 8 Mile tour on the day of the premier.
Instead of premiering the movie in Hollywood, Eminem wanted it to premier near Detroit (where he grew up and where the movie is based).
Em selected MSU as the campus to have the show, which was the college I was attending then. Me and my rap-crazy friends waited for hours just to get the tickets because we heard Eminem was going to be watching the movie in the theater.
So we get to the show and the entire campus is there, limos, famous people, people drunk off their ass... it was something to see. [this was the height of eminem's fame btw]
We got in the campus theater (which was completely packed), and the movie started. When the movie was over, the credits started to roll and the music playing was Just Lose It.
Then to all of our surprises, the screen started rolling up, and smoke and lights appeared from underneath. The entire place erupted. People were jumping over chairs and even other people to rush towards the stage. People were getting trampled, everyone was screaming, it was absolutely insane.
We got up as close as we could, and sure enough, Eminem came out and put on a full concert for everyone inside. Apparently not even the students working at the theater knew about it.
Needless to say, that was one of the best shows I've seen. Not only because it was one of the best surprises, but because the quality of the show was top notch as well.

the incident was also a special on mtv that aired around the same time of the movie release.