One nice is experience I had with xoops is that I was able to install the exact same module version (it hasn't been updated) I used back in 2005 on the latest release of Xoops. It still worked fine, many revisions later.
That won't work in joomla, trying to upgrade your site from 1.0 to 1.5. I bought a car rental app for joomla for $75. It came encrypted! I had no way to add a different make of car.. they didn't have Chevy, because the developer was French. After all the hassle, explaining to the customer, "no you can't do that" I would have been better off just writing it from scratch.
Other things I like about xoops is its "block" concept. Where you can create a new html block and specify what pages it appears on. For example, a disclaimer at the bottom of the site for a lawyer. But, say you need a different footer on the "pay your bill" page. Just create a new block.
It's not perfect. I would love to see a little more fine grained access restrictions such as a system of "locks and keys". I'm also a little bothered that they chose to take a joomla-esq approach of separating admin tasks. It used to be that you clicked on a module in admin, and you could edit all the functions related to that module. Now, in the admin screen there's a menu for modules and a menu for preferences. So, you edit everything for the module by clicking the modules menu, except the "preferences".
However, the cool thing is that because of the abstracted object-oriented design of xoops, the same module that worked on the old system still works on the new version of xoops. "Preferences" are just in a different place now.