80% is run of the mill stuff (such as explaining why we can't accept a newspaper clipping from the '50s) but there are some interesting people and organizations writing in that you probably would never get to correspond with otherwise (particularly those times when you write to the "official" address to confirm they are who they say they are). I have dealt with politicians, musician, artists as well as folks with unusual old family photos and companies trying to release promotional material but not quite understanding how Commons works. In the process I have improved my understanding of international copyright quite a bit. If you can stick to patient politeness in emails, then I would recommend offering to help out and giving it a go, the number of enquiries you deal with is entirely up to you and how much free time you want to devote to it.