Is It Appropriate To Bring Your Own Food Containers To A Restaurant?

I love this and wish I would have thought of it but my friend just turned me on to bringing my own to go boxes when dining out.

My friend and I recently dined together at a local restaurant and when it was time to go she brought out a metal container to take her food home. I like the idea that she wasn't wasting the restaurant's containers and she was being environmentally friendly. But on the other hand, it felt a little funny considering we were in a really nice restaurant. Is this a normal thing nowadays? I have never seen anyone do this until just the other night.

Some people think that styrofoam has chemicals and it leaks on the food that is what my friend said anyway and why she chose to bring a metal container with her to the restaurant. Do you think it is a good idea to bring your own to go boxes to a restaurant?