I worry so much about the nutrition aspect of CF. When we were looking at how to add calories to our son's diet we decided that the food must be high quality for the body to use it (Lauren mentions organic and it is something to seriously look at). There is plenty of science backing this up but it makes common sense not to put toxins in our body when we have a choice.
Convenience foods are loaded with processed ingredients like corn syrup, additives, toxins, preservatives and many other things you can't even pronounce. Why would you wish to give your body more challenges when it comes to digestion? These substances create a pro-inflammatory environment, which offsets the balance of good and bad bacteria in the gut, cause other systems to become inflammed (the gut is physiologically and energically connected to the lungs and sinuses), and taxes the immune system as it must work harder to detoxify itself from these substances. With a little planning these things can be avoided and you'll save alot of money on food.
Supplemental drinks are loaded with crap and will do a number on your blood sugar all day long. Focusing on "long-burning" foods (like nut butters, varied protein sources egg, soy, hemp, beans and healthy oils) and eating small high calorie meals throughout the day will help stabalize blood sugar and maintain weight easier.
Also, working to balance the flora of the gut will aid in digestion allowing the body to absorb and use nutrients more effectively. Probiotics are a must in my opinion for any CFer.