I'm really curious about fisting also. It always looks like the bottom is practically on another plane Of existence, they are so blissed out in pleasure. Or that it just takes you to a different place in your mind with all those pleasurable sensations. It's something I'm interested to try if I meet a top who is into it and I trust to take care of me through the experience.
All the diverse descriptions of fisting in this thread are more or less accurate. It's intense and it does send the bottom to another sexual plane of pleasure. When I've topped it's about getting really inside someone to an extremely intimate level but pumped up by the power you have over their feelings. Your arm becomes a cock that never goes limp and is bigger than huge.
The last quote above is so true too. It's not just about intimacy it's about trust! The bottom has to totally trust his top - with his life.
It's also best done by working up from a very slow gradual entry to a more rhythmic sliding before attempting the thrusting or punch fisting seen in movies. People like Matthieu Paris are professionals who get fisted almost daily and have complete control over their muscles down there. This is not for the novice. However even the first time can be a total trip with enough patience, poppers and lube. And as someone said above, it's even better with a cock to suck on at the same time.
A good FF session can last for hours if done right and another great thing about it is a good bottom can also make a good top because he / she can imagine what's going on inside there, so it's a good flip/flop sport.
Finally, with practice and a flexible physique you can fist yourself, so you can play solo too!
The major downside? Not enough available partners willing to try it.