It depends on the airline.
Most airlines are 20,000 Starpoints=25,000 miles.
LAN is 20,000 Starpoints=50,000 LAN
Continental/United are not worthwhile as 20,000 Starpoints=12,500 CO/UA but there are plenty of other Star Alliance carriers to chhose from to travel on them. Or you can trade someone Starpoints for AMEX points if you want CO miles.