I've been bottoming for many years and can say that there is no one, uniform sensation; much depends on my mood and the skill of the top. The best feeling is when his penis hits my prostate just right and I scream in ecstasy. There is no better feeling on earth than being taken by your man and having that intense pleasure from prostate stimulation. This is the most special moment - at the first contact with the prostate. I know that I'm going to have a really good time then. However, if the top has a smaller penis that jabs at the prostate, I scream from pain; it is not a pleasant feeling whatsoever and I try to accommodate by switching positions, which works sometimes. I am very tight and, if I am not stretched out, the initial insertion is sometimes painful. I have him withdraw, wait about 30 seconds and reinsert. It always works. I like to start slowly, usually with some deep kissing and getting fingered. Then, the sex should build up until, at the apex, I just surrender to him and let him take his own pace. If the feeling is really good for me, I will thrust back as a signal that it's okay to pump harder. I want him to experience the best fuck he's ever had on his own terms. As far as positions, if he is big, I like to start by having him lie down and me mounting him. That way I can control the speed of insertion. After it's comfortable, I will do whatever position he wants, although I prefer being on all fours with my chest on the mattress or floor, ass up. This usually assures maximum penetration.
But, the best feeling is always being fucked by someone you care for and feel safe with. There is no substitute for the intimacy, although, sometimes, I just want to feel like a slut and experience someone whose name I don't know and will never see again. That can be physically, if not emotionally, satisfying.