Before I start, I want to say, I know this doesn't happen every time. I just wanted to share my experience.
I decided to take advantage of the "press only" service the first night we were on board. I hate to iron. I had one pair of DH pants, 3of DH shirts, and my DD long formal taffata skirt. Everything came back the next day for less than $10. However, when I was hanging them in the closest, I noticed a small spot on my DD skirt. Automatically, I rubbed over it with my finger, and it smeared. I took the skirt to GS and asked them about it. They said they would clean it w/o charge. Fine, I thought. I was supposed to have it back by dinner, since it was formal night. Good thing she had something else to wear. The next day I called GS, and they told me there had been a little problem removing the stain. They told me it was paint! While removing the stain they put a slit shaped hole in her skirt and the color had changed in an area the size of a golf ball. They credited our account for the laundry service, and the value of the skirt, but the fact remains, my DD never got to wear her new long skirt, before it was ruined. We still don't know where the paint came from.
My point is the "press only" service is great, as long as you're not emotionally attached to any of the items. Once they leave your hands, it's hard to say what might happen or where they'll end up.