I've been fisted by a person twice, he had "small hands." It was a lot of fun. He had not really done it before and I had not been fisted before, my ass up on a pillow, and him working his fist against my asshole until it slid in, it felt extremely good and was extremely relaxing.
We went at it very very easily with lots of lub, it was not rough or anything like that. Then, I've put rubber fists up my ass a few times, that was a real turn on too.
I can certainly see where it could be very habit forming. It's fun getting past the width of the fist, and then feeling the asshole close around the wrist.
I've also tried it on my own. I can get four fingers and a thumb in, but I've never gotten my whole fist up my ass. I get to my knuckles and it stops there, I just can't seem to get that extra bit of stretch so it can slide in. I would love to feel my asshole closing around my wrist. I find all of this very very hot, I am really into my asshole, so to say!
I've gotten into estiming my asshole almost every night, I like ghost fucking. My asshole is always so hot, that's how I keep it all under control.