Thor's first owner had Thor in clothes almost every single day (+ collar, not harness). He always seemed perfectly happy running around, though he would scratch like crazy. I asked her about it once and she told me that another coworker had brought in some kind of doggie itch relief cream because she saw him scratching so often.
At some point, she stopped putting certain sweaters on him, and voila, he stopped scratching.
Anyway, when I got him, he was really good about putting clothes on, and he'd even help you get stuff to fit. Now that I've had him for a while, he seems to have gone in the other direction. The other day I took him the beach and it was quite chilly. I had brought what I thought was a really comfy, stretchy, form-fitting sweater for him. I tried to walk him on the beach, but every ten feet or so he'd stop and try to turn around. I took the sweater off him, and bam, he was a different dog. I'm still trying to figure it out. Maybe he's more used to sweater vests?