Nucleus CMS is not particularly sexy. Nor is it packed with features that are shining but that nobody uses anyway...
I use it for several blogs (yes, it's dedicated to BLOGGING), many of them are multiblog (integrated feature. No need for a complex manipulation: Wordpress users, you hear that?).
After using wordpress, Dotclear and Serendipity, Nucleus is the FASTEST to load and pages are generated at the speed of light.
As if you were in a wiki, you can add or edit a post directly from the interface of your blog.
I'm no technician... But I managed to personalize my blogs very easily. The template system is very simple to understand and to use.
It has a strong COMMUNITY: the forum users are always answering fast and efficiently.
Not so great:
lack of "new" plugins. All of them work fine, but I used to doubt their future.
Lack of more modern skins... Although you can personalize them easily, don't forget Nucleus is focused on CONTENT. The appearance is secondary: it's good, but a bit old fashioned.
I fell in love with this CMS after 2 days of trying it. I migrated all of my sites into Nucleus.