It varies with the man. Most of the physical pleasure from anal bottoming comes from prostate stimulation, and that response differs greatly among men. Some feel the pressure as intense pleasure, some as intense pain, some as neutral, and for the rest every degree in between. (Prostate pain can be a warning sign of infection, inflammation, or other disorders that warrant medical examination)
Often the pleasure is masked at first, due to the pain of the distended sphincter muscles, and from friction. Pleasure will become more noticeable as the distracting pain from these causes subsides over time with regular anal practice. Some men can orgasm from the bottoming alone, while others need the help of masturbation to ejaculate, and still others won't even become erect when mounted.
Anal pleasure also contains a psychological component, which likewise varies with the man. If you like the idea of penetration, your perception of pleasurable sensations will typically be greater than if the act repulses you, and be less dependent upon the lack or level of physical pain. (Excepting those individuals for whom pain may itself be a sexual stimulus)
I happen to get little direct physical pleasure from being a bottom, but can enjoy sharing in my partner's pleasure when he acts as the top. Performing as the top myself stimulates me enough to cum inside a condom.