I feel I have to resurrect this thread to offer my own experience.
Sorry if it gets a bit graphic, I will try and keep it fairly terse.
It’s all in the name of scientific exploration 
To test the feeling and result of a proper prostate massage I’ve attempted to do it several several times using various implements. Nothing ever came of it, pardon the pun.
Usually I would feel something pleasurable, and at times it would start getting really good, but for some reason it never got past that point, and it seemed the area would get insensitive and hard to stimulate again if I happened to slack for a minute or lose concentration.
Then I bought a curved implement, having been informed through research that it was the best possible tool for accomplishing the task. After a couple of tries which were also unsuccessful (success being defined as producing ejaculate fluid), I had my partner try. Somehow she managed to find a highly pleasurable spot with her fingers, but tired from the strain before anything earth shattering happened.
Now I had a much better idea of where to try. The area producing the strongest sensations wasn’t as far in as I previously thought, but I still wasn’t sure exactly where or how I’d find it on my own. On my latest attempt, it seemed a repeat of previous attempts, not particularly arousing or pleasurable beyond a certain point, up until I almost gave up and tried even less deep than I thought I had to, mostly so I could know for certain that it wasn’t the spot either.
Unexpectedly, the motion was intensely pleasurable and almost immediately produced a strong erection. Within a short while the rhythmic stimulation caused a full manually controlled ejaculation, with more ejaculate than normal. Interestingly, even though I tried to force the ejaculate by tensing my PC muscle, it didn’t have nearly as much effect as was accomplished by the tool.
Also interesting, while extremely pleasurable and easily comparable to a regular orgasm - though lasting much longer, there was no refractory period at all. The erection was still present and none of the mental/physical feelings that typically accompany the refractory period were present either.
It seems that being at least slightly aroused or erect helps the process considerably, at least in my experience, possibly by enlarging the area internally and making it more sensitive or plain easier to hit.
However, I’ve seen several people in online videos accomplish the same effect while being entirely flaccid, so it doesn’t seem to be necessary.
In my experiments I have not once experienced any pain or unpleasant feelings. I think being relaxed and not having any kind of hang-ups about stimulating that part of the body is important for a good experience. People with misgivings or some delusion that it’ll make them gay probably shouldn’t try.
It may take a bit of practice, as I can attest to, but all the fumbling around certainly paid off.
Good luck to anyone finding this post via google.
It’s something everyone ought to try at least once, in the name of scientific progress 