Chemical castration basically involves giving the male offender female birth hormonal birth control at very large doses. This differs from mechanical castration as does not sterilize the offender and the effects are reversible. The birth control hormone is progestin and it "quells the sex drive of offenders", presumably by lowering the testosterone levels in the blood stream.
Although the possibility of getting some serious man-boobs (gynecomastia) adds embarrassment to the punishment, some had a problem with the concept of chemical castration. Obviously there is a large violence component of the act of rape and this treatment suggests that sexual desire is to blame for the violence. If you can eliminate the sexual desire you can get rid of the violence. This seemed unexceptable to some, in that their perception of rape was that it is a violet act, not a sexual one. But of course high testosterone levels are linked to also linked to violent behavior, so perhaps it is two birds with one stone.
I did some quick searching for the efficacy of chemical castration on recidivism rates and it seems like the data supports castration as an effective means of preventing reoffending (1,2).
BTW - Mike Tyson, the boxer/convicted rapist (pictured), was NOT chemically castrated, his voice has always been that high. To retain a high-pitched voice you have to be castrated before the onset of puberty.