I've done quite a bit of research recently on these types of "vacations". "Mywildvacation" seems to be the best if you're talking latin american girls. For more info, just read the forums on the site. There are several "trip reports" that will give you the info you want. Bottomline is, if you want a relatively hot, friendly latin american girl hanging out with you 24hrs a day, and be able to change girl every day, and have a sun/beach environment, this is probably teh best choice. Some other spots (in the DR) offer hotter Eastern European girls for twice the price, but from what I've read, they are money hungry and not very friendly.
I've read about "medellinmansion" as well. This seems to offer another type of experience altogether. Girls are very hot, paying by the hour (or for the entire night) isn't a bad idea for those who don't want to get stuck with a girl 24 hrs a day. But since Medellin is land-locked, no beaches here. Girls at the mansion might not be underaged, but not so for those in clubs and bars. So you have to be careful there... Anyone over 18 should have a "cedula" (ID), so if the girl doesn't have one...
I might try one of these two places in October or November. Haven't decided yet.