Conditions for it to be valid:
1) The rule and authority is Islamic.
2) There are 4 male and realiable, sane honest witnesses that witness that the actual act of sexual intercourse (not just them sleeping together but actual penetration) is taking place.
3) The adulterer is married to someone else.
4) Or the adulterer confesses to someone in authority and hands themselves in.
It is almost impossible for all these conditions to be met, so actual punishments would be very rare, the law is so tough because it acts as a deterrant when people are brought up in a society where such laws exist...they know in their hearts and minds that it is really bad so they are unlikely to even go towards such acts.
Can this law be applied in a land that has newely been brought under Islamic rule? Or is it better to wait until people are brought up to speed and educated gradually? What if the Islamic authority and rule is lax in applying other parts of the shariah, for example they allow Capitalism to function unhidered with its riba and fraudulant transactions and allow the rich to steal using the pen and other sophisticated means while they punish the poor and adulterers in this way...is rajm still valid in a situation like this? How can sinners who continue to sin big, punish others?