Dear lord! OK everyone is different and we all like different things.
For the people who enjoy it (like me) it is an indescribable feeling.
I am an odd ball in that I like cocks of pretty much any length, girth, curve, what have you. They each offer their own unique pleasures.
Yes I do have my limits of what brings me pleasure. I have had mind blowing hands free with a four and a half incher, thin side, sharp upward curve and standing straight up when hard. With legs over shoulders, he battered my prostate like a damn speed bag and got me off in a way no one has since.
Next memorable: Mr. Average. very slight up curve, average girth, upper average length. Worked wonders in a variety of positions, and caressed my insides in a whole different way then the other one did. Never did make me have a penile orgasm, though did lend me my first non prostate specific anal orgasm..
Current? Larger than average, round 7(nbpel)-7 3/4 (BPEL)real inches, thick enough t need a magnum xl, up and left curve. Some positions push more on my endurance of the act then pleasure (though i get off on that) where as others are as if we were designed for one another. A few simultaneous Big O's.
Now I did not intentionally move up the size ladder. In fact if the first guy came into my life I would have to have him inside me again (current guy knows this and is completely fine should the opportunity present itself)
In fact, before Mr. Average, I had an experience a few years prior with a truly well hung man. That gave me the upper limit of what i like. He was 9 inches, thick and I was uncomfortable the entire time. I had, a few months prior been with a bloke of ridiculous girth, and all that did for me was cause me discomfort, pain and then a numbing sensation.
The smallest was just under 4 inches and poked and hurt. I had a guy at almost 8 inches who felt like he was stabbing my guts (his dick was ramrod straight).
So don't assume that any of our experiences will be in any way indicative of yours. You will find out if you like it, and what gets you there.
Enjoy the experience.