Plastics vary in their tendency to leach into food. I wouldn't use most cling films in a microwave, for instance; they aren't designed for it. Oven bags are intended for that environment, though; I think they're probably safe.

With that said, I think Pyrex and other glassware is a much better choice... excellent for cooking, and environmentally more appropriate, since it's usually re-used and it's not made out of increasingly-scarce oil.