I previously had lights overhead. We put in side lights based on what I read here and other places. My sidelights caused glare on my eye-glasses and they were hot. They raised the temperature in the bathroom too much.
I replaced the bulbs in the side lights with compact fluorescents and it helped a LOT. Both with the temperature and the glare.
But I still don't see a great advantage with the side lights. I agree that they do light the front of my face better, but I am a guy and my most critical lighting requirement is when flossing. Overhead worked okay for me.
I would say to make sure the sidelights are far enough to the side so they aren't too much in your field of vision. Make sure there are other lights that illuminate the room brightly enough to reflect around and eliminate shadows as much as posssible.
Use multiple stages of lights or dimmers so you can turn on as much light as you need depending on whether you are just washing your hands or applying makeup.

Our 50's bath had a mirror with built on side fluorescent tubes- I hated the brightness in my eyes, especially when getting up early in the winter for work! I turned them off, DH turned them on!

If you have light from multiple angles (above from a few recessed or above mirror fixture) you will diminish shadows, what is the real issue when lighting a vanity mirror.

A separate switch for the above and side lights can also leave the option for others who prefer that light.