I have had one for years, since I was in college. My roommates didn't know much about CF, and I am on meds and have allergies.
I use Medic Alert and they have some cheap but cute designs. I prefer Medic Alert because you can update your info online and keep it up to date, and it allows first responders important info you need emergency care. I have two silver colored bracelets, one with a pink emblem and one with a blue emblem. They also have designer styles, fabric bands (great for kids), etc.
If you are on the fence - think about what in your medical history would it be important for someone to know should you be unable to speak for yourself. Not every person with CF may find the need for one, but I personally do. <b>If any part of your condition(s) or meds might effect or be effected by a common treatment for let's say a broken leg (anesthesia, blood products like thinners or coagulants, drug allergies, dextrose via IV), then I would suggest you consider it.</b>
It's so important to keep info current, though - meds, conditions, doctor info, allergies, etc. Updates are FREE and unlimited, and you can print out a piece of paper to bring with you when seeing a new doc or having surgery, etc.