XOOPS is an Object Oriented content management system matured to the point where it is useful and has some features worth considering. I would not put XOOPS ahead of Drupal but I will point out that XOOPS already has automated module installation which was missing in Drupal 6 and only part implemented in Drupal 7.
The range of available add-on modules is often the deciding factor in a CMS. If the CMS has the modules you need, you can complete 90 percent of the work in a couple of days. XOOPS has hundreds of add-on modules. Drupal has thousands of add-on modules.
OO, object oriented, code is good. The OO was put in the name XOOPS because, at the time XOOPS broke out from a previous project, OO was rare in the CMS market. Now there are plenty of systems using OO but not necessarily consistently.
The XOOPS code may not be the best code in the world. It was a step forward at the time and it was also a conversion of an existing system which usually locks in design limitations from prior to the conversion. XOOPS has had enough subsequent development to let the developers rebuild into a clean system. I tested XOOPS prior to 2.5.1 and found a number of problems with the design. XOOPS 2.5.1 uses a new theme system that may be one of those big cleanup steps.
Almost everyone uses MySQL. Commerce oriented sites use PostgreSQL for transactions. MySQL now has InnoDB for transactions. XOOPS uses MySQL with no option to use PostgreSQL or anything else. XOOPS still supports the old MySQL 4.1 at a time when people should be planning to switch from MySQL 5.1 to MySQL 5.5. That makes XOOPS equivalent to Drupal 6 and behind Drupal 7.
XOOPS handles multiple languages, multibyte characters, and the display of text from the right to the left. XOOPS appears to have everything you need to set up a web site for any language in any country.
XOOPS used to have manual module installation similar to Drupal 6 and other CMSs. Wordpress gained an automated module installation system and set a standard. XOOPS now has an automated system looking as good as the Wordpress system. Drupal 7 includes a step towards full automated but Drupal users will have to wait for Drupal 8 to get the equivalent.
XOOPS recently switched off support for PHP 4. There is no roadmap for the variations across PHP 5.0, 5.1, 5.2, and 5.3 or preparation for PHP 6.
People choose Wordpress over other content management systems because Wordpress has many nice looking themes ready for download. Drupal has a lot of downloadable themes and some are conversions of Wordpress themes. XOOPS has 873 themes ready for use. Some look like conversions of Wordpress themes. Many look similar with slight changes in decoration. Based on a quick look through XOOPS and Drupal themes, Drupal must have more page layout flexibility.
XOOPS development is a little bit behind the market leaders in some areas, up with them in other areas, and is ahead of Drupal on one point. I would consider using XOOPS if working on a project where the current range of add-on modules does exactly what I want.