Answering for my SO - YES! I've always wanted to do this but hadn't really tried before and never thought I'd get to. But she was willing to try and I knew it would be something to work on - not just one day do it. So 3 or 4 times we worked with my hand, using a glove and lots of lube, but we never could get it all the way in.
Then one morning I was playing with her using my LEFT hand (I'm right handed.) I played a lot and she was VERY turned on and VERY wet. I started stretching her with 2, 3, 4 and more fingers. Eventually I tucked my thumb and slowly eased my whole fist inside! She had a massive orgasm and then it felt painful so I took it out. I never thought about it or checked before but my dominant hand is quite a bit wider and meatier and boy does it make a difference when every millimeter counts!
As for the experience, it was amazing! It is such a bonding feeling to be so intimately joined together. Doing something special and 'extreme' is such a physical and mental turn on. Feeling her whole body around my hand, squeezing so tight, seeing my wrist held by her lips... all amazing. We still do it although some days it gets too much so I dont get in all the way. Usually she's using her fingers or a vibe on her clit and we share her incredible orgasm together.
And just in case anyone's wondering, no, she is not all stretched out and she feels just as tight as ever when having vaginal sex later or the next day (she's usually a little sore right after).