We have a student working for us on comparisons between various LMSes here, and we recently moved on to Sakai. We've been looking a bit at installing it on an Ubuntu 9.04 and boy... is this a challenge. While there seems to be a few sources of documentation around the web (including on the Sakai website itself) on how to install it, they're all giving a 20 pages-long guide on how to install a specific version of Java (from sources), a specific version of Tomcat (from sources) and a specific version of Sakai (from sources as well).
Although not very reliable per se, the following article, written in 2006, has an interesting set of comments (part by me) about statistics about Moodle, Blackboard/WebCT and Sakai, and Dokeos of course. http://zacker.org/higher-ed-lms-market-penetration-moodle-vs-blackboard-vs-sakai