AC is among my favourites and one of the few games that have really appealed to me in recent years.
I have all of them except #3, but I think I'm way too much into the Renaissance, Italian theme, together with the Arabian world that AC I brought. I'd really like it if they continued along those lines, but that may become repetitive, a mistake that many other game series in the past have made.
I suppose seeing how many Ubisoft have released now and how many more may be planned, it's a good indication of AC's popularity.