Most of the fincas are great! Just be aware that if you go to sample the nightlife, they are mostly located (with a few exceptions) quit far away from the nightspots or the beach. Make sure you locate the place on a map before you book it!
By the way, try
http://www.parador.es/english/index.jsp
You find the most amazing places, most hotels are historical buildings (starting from 11th century, most 15 to 17th century) who where converted into hotels already in the 1920's in order to preserve the historical character and not let them fall into ruins. They do not have that many places on Ibiza though!