I know Stainless steel is safe and I have looked into it, but right now I don't have my schedule set up to prepare and heat my food before I leave for work. The way I do it, I can have my meals packaged and ready in my fridge/freezer and then I microwave them at work. I do like pyrex, but just haven't got around to investing in any for lunch ware.
I also really, really like my food to be hot (temperature wise. Spicy is good too though) and not lukewarm. But we'll see. Once I start medical school in the fall I might experiment with some sort of thermal system and see how I like it.
Edited to add: I am aware of the many various options and safety concerns for portable lunches, btw. My current inexpensive system is largely because I'm working in an infectious diseases lab for a year and tossing most of what's been coming to work with me at the end of my year (which is soon. Yay!).