I had searched and searched for months to find a high quality classifieds script, only to be disappointed time and again. Price was not a big factor. In fact, I was so discouraged I had decided to hire a programmer to write a custom script from the ground up. Then I found 68 classifieds.
It has a clean, professional interface, very flexible, highly customizable and the price is simply amazing. Add to that an incredible support system and you have the finest classifieds product out there. I know. I've owned a few from the most popular companies (and paid $$$$$). I found them all to be very inflexible, especially considering the price I paid.
One factor in their success is that the 68 Classifieds product is heavily driven by customer input. They continually shape the product to the needs of their user base. It just keeps getting better and better.
No product can be all things to all people, but 68 is the most flexible system I've ever seen. I have done all of my customization with the designer version using the template system, even some fairly major stuff. If your needs go beyond that (that is, needing changes to the core) you could get the developer version, hire a someone to customize to your liking and still pay a fraction of what others are asking for their stock scripts.