Any series that releases a game every year usually isn't worth the full price tag. If you disagree with me, consider Halo vs. CoD as a generic example. One of them rewrites the engine each game, even replacing the old one; the other rehashes the same engine and throws on a [i]seemingly impressive[/i] graphical improvement, which levels out a few games later.
Um. Nobody really rewrites their engines, that's a ton of work, and takes a team a year or two to do. Most "rewriting the engines" are engine rehashes, taking out parts of the old engine and optimizing it and such. Halo included.
Either way, it's still work being done to the essentials. I just found that with the AC series, it was virtually playing the same game 4 times in a row. But that's just me.
Definitely not, and I suppose it all comes down to personal preference, which I never have had for the series. I can see their value, but they're not for me.