I have enjoyed diary style fiction of various types. I am not a huge lover of apocalypse type books or last person alive (except for the cockroaches ) books but could see how that would work as a diary. If it is a diary I like a bit of suspense so don't want to know all the details of who, why , what, where and what will happen next untill the end...although I apppreciate what goes before will hint at a final outcome.... The reason for the person being the last person will have to be explained and the author could choose between getting it all out there at the start or a slow reveal over the whole book/story. A diary would be a good way to explain the back story and set up the ending... I could go on but I am sure you will get more interesting answers from others.