The pirate theme appeals to me more than other AC games but I can't bring myself to buy sandbox/open world games. I just end up dropping them after a few hours because of the lack of structure and focus.
I still believe 2 and brotherhood are the best, but ubisoft completely redeemed themselves with IV
I like what they did with the game, and how at least they tried to save the main story.
Still, they need to improve many things, like the combat system
Pretty much my feelings as well. I was ready to drop the series after III but IV is just complete redemption. Very fun game, and not just as an AC. Even if you aren't into the core of the AC games (the whole assassins vs. templars plot) there's tons of neat side stuff to do and one of the few sandboxes that doesn't feel like a chore to fully explore. Anyone who's interested in the golden age of piracy or open sea exploration should play it.