It depends on what you mean by an intro. If you mean your username 3D Animated to spin around and flip with dubstep music in the background, no, it's not a good idea. If you mean welcoming your viewers to the video with a quick bit at the beginning, then yeah, go for it! If you mean over-editing one of your clips in your video to be funny or entertaining and serve as an intro to the video, then that's what I do, and if your content fits, then it will do wonders to your audience retention. For me, I doubled my average audience retention by doing it, so it was a definite success and I'm planning on doing more like it.