This is one of those WHS's where the reason for inscription is actually more about something that is hard to physicalise... so they just create a reason to use some buildings to represent the idea. As a result, I found it rather unimpressive.
Obviously the history of Martin Luther is extremely significant and the bonus of visiting the places included within this site is that you learn a lot about it. It wasn't a topic I knew a huge amount beforehand, so I appreciate that. However, I felt more like I was in a series of museums rather than seeing something of actual historical and cultural worth.
Personally, I found Wittenberg to be more enjoyable than Eisleben - but it was also a much more touristy town and there were a lot of people around. Still, everything in both sites is done very well and if you know what to expect, I don't think you'll be disappointed.