I can orgasm from anal, without penile stimulation, with relative ease. When I'm very much excited and aroused, I can cum from penetration alone, from the in-and-out motion, by using a dildo on myself... there are times that I just really crave anal, to be penetrated, and using a bigger toy makes me orgasm from that alone, without ever touching my penis. The stretching and that completely filled up feeling creates an extremely intense pressure that, quite literally, just "forces me to cum". It's strange really, but I cum very quickly with that sort of play (in fact, there were times that I have orgasmed from the initial insertion, right when the dildo went in balls-deep... it was quite intense, I was very much excited and felt that I had absolutely no control). These orgasms from anal play, without any penile stimulation, feels incredibly intense. They build up, slowly but very surely, and when I finnaly orgasm it feels like my entire body is climaxing, not just my penis. My whole body twists and turns, on the verge of literally exploding, pulsating from the inside out. I think that one of the main differences from penile orgasms (at least for me, and comparing from masturbation only) comes from that loss of control... something (or someone, if that's the case) is entering you, penetrating you, fucking you, conducting your pleasure and taking control of your entire body (mind, soul?); your excitement just builds and builds until it finnaly collapses, rendering you completely helpless, forced to cum. You are left feeling fucked, violated, satisfied. Pleasured.
But here is an interesting thing: I dont like too much direct pressure on the prostate itself... I always seem to find prostate massage with my fingers a little "too much", almost painful even. I prefer indirect stimulation, from the perineal area and from anal play or penetration. The simple, in-and-out motion, without even aiming for the prostate, feels much better to me.