Margaux took to it right away - she was probably about 2 or so. Maybe that was partly because the harness was very light, like a thin cord (I don't think I'd recommend this, although I didn't have any issues), but probably more because she always wanted to go outside. I think she figured that if the harness was the only way she was getting out there, she'd put up with it. I brought her to the door and opened it a little before I put it on for the first time. She resisted a little, but I opened the door again so she could get a whiff of the air outside and then finished attaching the harness.
Yep, I'm in complete agreement with MowMow about what it's like "walking" a cat:
Only once did she actually let me walk her. The other times, it was me standing there while she sniffed a bush for 10 minutes, moving forward a few inches, standing again while she sniffed a second bush, etc. Occasionally, it was being nearly yanked off my feet when she caught sight of a chipmunk. And then it was growling and hissing when it was time to go back in.
Celia would not have anything to do with the harness and got totally freaked out when I tried to put it on her. I tried a few times, but she's always been skittish and I think she's actually afraid of the outdoors, so I didn't push it.