I have a mirror for models to put the makeup on, if needed. But what I found that it acts as a really great main light for portraits, also.
The light is great, catchlights are very interesting. The main issue is finding a good angle, so you don't catch yourself in the reflection, also.
I use a long focal length (85-100mm) and usually shoot pretty open (f1.8-f2.8), so I can isolate the model almost completely.
I also sometimes add rim lights to give the model's hair a little more definition.
Here are some more examples from the mirror mirror on the wall :)
If you have any additional questions about the gear used or the shots, feel free to ask.