Jeff, it can take a VERY long time to get a website ranking well these days, and especially in such a competitive industry such as holiday rentals.
I always advise people to search on Google for terms such as:
"villas for rent costa blanca"
"villas for rent in torrevieja"
"holiday rental villas in ..."
etc.
The sites that come up well should be the ones to target as they will get very good quality enquiries coming through as these search terms are very specific.
We have a free rental section on the site and it's proving very difficult to get our holiday rentals to rank well, it's so competitive, although we will get there...we always do
Our long term rentals rank really well and we get loads of long term rental enquiries every day. Search for "long term rentals ....." and we should come up.
So in answer to your question....I think it depends on how much marketing they do...but you can't beat free search engine ranking for generating enquiries, so those sites that rank well are definitely the ones to look into further.
I could go on...but I won't!
justin