They are not ideal but I believe they are useful. I use a resistance band during my back workouts when I am no longer able to complete a chin up/pull up on the bar. I eventually plan to toss the bands away but am working my way up to that. I recommend them for chest workouts.
I think they are awesome for a definite change of pace. I often take them when I travel just because they are so easy to travel with. You can google many resistance band exercises and come up with all sorts of stuff.
You can't out-exercise poor eating habits.