Breastfeeding is not contrindicated for mothers with CF. There are some special considerations that others have already mentioned, like calories, sleep, etc.
Personally, having had to use a bottle a few times while we were nursing. I would breastfeed any and everyday. I found that I actually got MORE sleep because of the hormones released when nursing. I kept my duaghter in our bed so I didn't have to get up to get her and that worked very well for us. For others that don't sleep so well sharing their bed with a baby, there are lots of other options that make nursing and sleeping easier.
Calories, I had trouble at first, but once I got my diet on track and started some quality supplements and got into the routine (establishing your milk supply in the beginning is VERY important and sometimes your body initially burns more calories and makes more milk than it needs to. If you give it time and have in there, shortly it will adjust.) things went fine.
Please do read the breastfeeding fact pages and my story on <a target=_blank class=ftalternatingbarlinklarge href="http://www.geocities.com/MurrensNatureMama">www.geocities.com/MurrensNatureMama</a>. That will answer a lot of your questions, but feel free to email me with others if you like. I'm on here infrequently, so email (<a target=_blank class=ftalternatingbarlinklarge href="mailto:akaeg@netzero.net">akaeg@netzero.net</a>) is your best bet.
I also encourage you to go ahead and get involved with a La Leche League group in your area. My LLL Leader was a life-line for me. I don't think we would have been successful otherwise. Feel free to print off anything you like from the site and make sure you tell your docs ahead of time that you are planning to do this and would like their support. That will make it easier on them and you.