This manga, like it seems a lot of manga has interesting concepts, but ultimately the implementation felt like a mess. I like the concept of a ghost coming back in time and how that could interfere with the development of a relationship, and furthermore where such a plot could lead. I also found it interesting how the characters of the past and future version were so different, and thought that that was a good set up for solid character growth. However, as it went on, none of this was materialized on. The rules and mechanics of being a ghost and everything surrounding that were arbitrary, and hence the plot felt random as well, all the way up to the ending which felt incredibly unsatisfying in that it felt rushed and also in that the memory aspect felt largely unnecessary. The character development and relationship development was random too, in that it barely happened, and things just ended up falling into place. On top of that, I'd have to say that while the premise was good, the characters placed in the roles didn't fit all that well. Also, the humor was some times okay, but largely was not even that funny. The art was also only okay. 

tl;dr: An interesting premise that loses its way really quickly.