They are very good.  Take your vegetables, which should be around  the same size and density so they cook at the same rate, wash them, dry them and cut in uniform chunks.  Toss into a food storage bag.  Drizzle some olive oil into the bag along with some salt and freshly ground black pepper.  Close the bag,  shake or squish around so the vegs are coated evenly.  Pour out onto a foil lined baking sheet in an even layer.  Roast at 400 degrees until fork tender or until your desired degree of doneness.  I like a little char on mine.

I've used potatoes, sweet potatoes, carrots, parsnips, broccoli, cauliflower, sugar snap peas, cabbage wedges, Brussels sprouts, onions,  and bell peppers.  I like them all.