When taken properly, meaning the pill is taken at about the same time every day (there's about a three hour window in which the pill can be taken for it to remain effective) and no pills are missed, the pill can be over 99% effective.
Do note, though, it is recomended that couples who are going to use hormonal birth control as their only form of protection should be and stay in a monogomous relationship, and have TWO full and clear STI screenings 6 months apart before stopping the use of condoms. Then, in good practice of sexual health, they should continue to get full sTI screenings every year after that.
There are many couples who do use the pill or other hormonal birth control as their only form of contraception (myself and my boyfriend included). It is a good choice for couples who are in a long-term, monogomous relation and who have had full STI screenings.
Just to be nitpicky, as well, when couples use hormonal birth control properly and have had the two full STI screenings, they are not having unprotected sex, their sexual activity is very much protected.
[This message has been edited by dailicious (edited 08-28-2005).]