I have a quite bit of experience with a modelling tool that some organisations use in conjunction with SAP HR in order to model their organisation structures. In my very limited experience, one of the biggest challenges seems to be to keep your HR data current, this is at least true for two large mining companies. Restructuring within the organisation seems to put the SAP HR team under quite a bit of pressure. That's unfortunately all I can contribute.
Off topic: How did you folks get into SAP? Certifications? I have quite a bit of Java development experience under the belt which as I understand it, is used alongside ABAP. It's a future career path I'm considering but I know very little about the SAP environment apart from some experience with SAP Solution Manager.