It's not that uncommon. It's not common, but most people seem to only be able to do it occasionally. It is much more uncommon for people to get off purely from anal every time.
From what I've read experience wise, none of this is remotely scientific.>>3271492
You don't need to cum from anal for it to be amazing. Once I start thrusting it pretty much feels as good as the peak of a penile orgasm, but for the whole time, which is usually 10 - 30 minutes before I get tired and jerk off.