I've done it once successfully and once not successfully. What an amazing feeling, I really had no idea. The lady where the mission was accomplished was experienced and relaxed. We went real slow and she LOVED it. Can't imagine how it felt for her, but it was a very cool sensation for me as the giver. It gets so wet (used plenty of lube to "grease the skids", but the natural pussy juices were just gushing. She was a squirter as well, so it was an intense experience. It's hard to describe how it feels to have your whole fist inside a woman's pussy, but it's unique and very erotic.
I liked it so much I wanted to try it with my ATF. She's a mature lady who has an open mind about stuff, but was a bit nervous and hesitant to try it. Again, plenty of lube and I went real slow, but she never relaxed enough to allow me to get my whole fist in her pussy. She wasn't enjoying it, so I stopped. I'm not interested in making someone do something they aren't getting off doing and I totally respect a ladies right to set her boundries.
So it's not for everyone, but can be a mutually satisfying experience for both parties if done in a slow, controlled way.
Can't really imagine fisting an ass though, or being a receiver. The vagina is a pretty incredible, flexible part of a woman (hell, babies come out through it), so I get that it can accomodate something the size of a fist. The back door, not so much. But good on ya if you can make it happen and enjoy it!