I am looking at Classipress myself, but have not used it yet. My quick impressin is that Classipress is good for the the blog like look and flexibility, while Xzero and other look more like fixed, often Craiglist-like, sites.
I use the basic version of GeodesicSolutions.com software for http://www.VintageRadio.com. For http://www.73.BIZ, I use ClassiPress from AppThemes.com. Both are excellent scripts. Geodesic Solutions system is very easy to use with PHP being in easy to move and manage modules allowing a lot of use of good old, basic HTML. Modifying the ClassiPress code is a bit of work unless you are a PHP coder. They and Wordpress change code with some frequency which creates work. You DO want log-ins and capcha codes. SPAM can be bad and that at least helps some as do some free to use scripts. I believe that xZero is an no log-in script. I think that you will find that to be problematic. You can bet plenty of SPAM ads for SEO, web design, 'Pretty Dresses' (with no pictures), money making opportunities, sex stuff, and more very easily but you won't want it!