Wow, I was just thinking of starting a thread about this very topic.
yes, I've had bad experiences.

I have cystic fibrosis, and unless I'm on IV medication with a chair, I don't look sick from the outside all the time. Its a blessing and a curse.
I get dehydrated extremely fast, even in 15-20 mins. I can walk fine, unless I'm on IVs. I get frequent coughing attacks, need medicine doses at different intervals which take up several hours per day, and I can't stand still for long or even shorter periods. The max time I can be in disney is 4 hours. So with little kids thats not alot of time! But I need to get back to my room to do my meds and physical therapy. We manage and still have a fantastic time.
I get a letter each year from my docs to give to guest relations at disney. One year, I had an awful experience that left me in tears and actually left me upset for hours after I had gotten in the park. I gave the cm my letter and asked for my gac.
She looked me up and down, said 'you don't look disabled to me', and said I couldn't have the gac. I calmly told her to read the letter, and said I get a gac every year. She rolled her eyes and said fine but only gave it to me for 2 people though I have a child. So that means that my hubby, me, and our daughter wouldn't be able to be together at all on anything!!!! I was so upset. (bc I know she could put more than 2 people down! plus she saw my baby-was I expected to go alone on each ride for our trip? or carry my baby alone on each ride?)
I protested of course. She leaned close to the glass and said 'i know what you're doing. be happy i gave you this card for 2 people. this is for people with REAL DISABILITIES or terminally ill children, not adults.' I was SHOCKED. I asked her 'what are you saying?!?' She replied 'well, whats your disability?' (though I already gave her the letter!) I firmly told her that according to the disabilities act she had no right to ask me that, and I needed the manager. When I said the words disabilities act she seemed to get scared and said forget it, and was about to just hand me the card for 3 people. But I insisted on seeing a manager. The manager came out, asked me what I wanted, read my letter and gave me a gac for 6 people.
I was crying at that point, I think just from embarassment, and tiredness. I should have told the manager word for word everything that cast member did, but I didn't. I just left. It was awful and it ruined our day.
There have been several attractions where the gac just doesn't help. I think at times its been legitimate and I know there's times its been the cm at the front of the ride at that particular time. But 90% of the time we have good experiences.
There's only once I can think of where a fellow traveler at a ride made comments to us. But to be honest, I couldn't care less. I've gotten thick skin from women at grocery stores over the years who've called me a liar for using my disabled parking tag.

This is pretty common with people that have Cf. Its sad and a big frustration.
We're going to dw again in the fall, this time with our newest little one who'll be 6 months old at the time. We're soooo excited, but I'm also nervous about this aspect. We haven't traveled with 2 kids yet and I don't want to get sick or exhausted but also want to give the kids a wonderful time. I really don't want to be embarrassed again like that. Especially now that my daughter is older and understands more.