I find the bags impossible to keep clean, crumbs get into corners and if yoghurt spills it stinks. We're in Australia and lunches are kept in school bags which are kept outside the classrooms (really nice when it hits 40 in Summer!). I use plastic boxes with those cool blocks you put in the freezer. In Summer I also freeze cartons of juice which will have defrosted by lunch, and freeze frubes for extra chill, you may not need those though
I prefer the bags, as things seem to fit in easier. I do use tupperware boxes for the contents. I just put them in the washing machine when they get grotty, at the moment we've got a Thermos one and a Maisy Mouse one, they both go in the machine fine.