The audit feature is probably the best feature of ECM (which most programs have already). This feature shows you who has accessed, downloaded, and viewed certain files / folders. Their desktop application is also a great feature, allowing you to drag and drop files into ECM. The best way to re-organize files is through the ECM desktop application.
Security in terms of adding and removing privileges is terrible. Part of this is due to the setup of our current environment. If you want to update permissions for a specific folder, you have to push those privileges down to all files, which in some cases can take hours. When you attempt to push these permissions, a window pops up which cannot be closed or it'll stop updating permissions.
The groups in ECM don't make that much sense either. SharePoint has the right idea with groups. When I create a group, I should be able to enable users to create folders under that group. I shouldn't need special permissions to create folders.
Lastly, moving files is a pain. It's a difficult experience to re-organize files and folders. If you don't have the desktop application installed, it is not worth attempting to do. Drag and drop feature doesn't always work and sometimes ECM just fails to upload files with no error. Multiple people attempting to edit files, doesn't work. Multiple copies of the same files will show up sometimes.