I installed workrave-xfce from the PPA, and tried to add the workrave item to the panel. However, it does not show anything. On adding a single workrave item, the icon is grayed out so you cannot add any more, and the item in the panel items list indicates "Workrave (external)" but nothing is displayed. If I remove the item from the panel, the executable xfce4-workrave-plugin is still running as a process.
Running on Ubuntu 14.04 i386, with workrave 1.10.6.0-ppa2~trusty2

(Selecting the applet from the workrave preferences enables the gnome-style applet in the preferences area, but I'd prefer to use the xfce native one to avoid the issues with that...)

I just did a fresh install of Ubuntu 14.04 and install workrave-xfc from the PPA. Seems to work fine with 1.10.10. So you still have this issue?

It works a lot better now. However, the calculation of the size of the widget still seems strange - sometimes it was wider than the actual content, sometimes smaller. But it does seem to actually do something now :)