In my case, it is my husband who has crohn's. So, my perspective is probably a bit different. I do think that if I had known my husband would be diagnosed with a chronic incurable disease, I would have backed out of it, in the initial stages when we were just starting to go out. I think that's normal human instinct - after all, we are biologically programmed to select the "best" mate.
I think in order to give yourself and the relationship a chance, you need to be up front before the relationship becomes more than the friendship stage. But you need to allow the person to become your friend without scaring them off with the illness.
Would I walk away from my husband now? Heck, no! But I would have, if he'd told me, "I have crohn's, and it's an incurable disease, and it's chronic, and I have to take some pretty crazy drugs to keep it under control, and they may have to surgically remove parts of me from time to time, I can pass this on to my kids, cancer, blah blah", when we were just starting to become friends. But once you're friends, you should disclose the illness, and see if the person can be supportive, and still see you, and not just the illness. If they can do that, then, the relationship has a chance.
Also, age is an important factor in how people deal with facing this kind of information. The more life experiences a person has (and that usually comes with age), the more accepting they are of the cruel twists that life sometimes takes, and are a lot more able to deal with the spectre and reality of this disease. I am 31 years old, and I struggle every day to cope with my husband's illness . . . but I'm getting better . . . maybe by 40, I'll be more accepting of this illness.
That's my 2 cents.
Husband with Crohn's
Diagnosed March 2003 Ulcerative Proctitis
Diagnosed March 2008 Crohn's & C-diff, hospitalized 45 days
Canasa, Lialda, Remicade, VSL#3, Florastor
In Remission since June 2008
Stopped vancomycin for c-diff Jan 1 2009
C-diff free, until Sep 2, 2009
Fighting c-diff, I guess for life