OX has tasks, projects, documents, public folders, several admin interfaces and lots more.
You compare a solution that is able to store appointments and to handle mail with a full featured collaboration suite. Wow!
Right, OX does not have drag and drop (right now) but you can integrate OX in an existing infrastructure. This is important.
There are Debian packages available and Gentoo has a portage where you can install OX in 5 minutes with the services from the distribution. This is real integration in my opinion. BTW, OpenSuse has OX as a package directly included ...