MC's are like Country Music, Pizza, and Democracy. When they're good, they're great, and when they' aren't, they're awful.
My friend Max aka MC Duh, who does some hip hop and D&B MC'ing is really good. For one thing he lays out of the mix a lot, but when he's in he's good. The secret, aside from some verbal skill, is to listen and watch the crowd, so that when you get on the mic you're in the moment, and not a distraction from it.