I'm a top and I play around with prostate toys but have yet to experience anything like what I hear other people describe. Sometimes it does feel really good, but it's not mind blowing the way guys seems to describe it. I've also watched some vids of guys having anal orgasms and that is definitely not happening to me (the convulsions, flopping around, and whatever.)
Years ago, in college, I had an ex who used to visit every few months and we'd have sex for hours. I'd start by rimming him, playing with his hole, getting him really sensitive, and then I'd slowly begin to fuck him. He one time started shaking and yelling after I had been deep in him for awhile, but we weren't fucking fast, just gentle and long pushes in and out. I stopped because I had no idea what was going on - and he said that he felt like he had cum two or three times in a row (but he hadn't shot anything.) His dick went soft but as soon as I started moving inside him he immediately got hard again and was literally moaning out of control to the point where I had to stuff a sock in his mouth to muffle the sound (we were in my dorm room) - LMAO.
It was pretty incredible for both of us. He had a nice thick cock of his own which kept alternating between soft and then rock hard again, but he never actually shot a load - and after this happened, we kept fucking for a couple of hours - he told me he felt like he had cum 20 times. I don't think either of us knew what it was called or what was going on but it was hot. Have never had that happen with another guy since - maybe because most guys just want a quick fuck-and-cum and so there's not enough time for the sensation to build? Would love to find a bottom to help achieve this, again, though! It was hot - and also fun.