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It's a significantly different, more accomplished game, now. The Ezio series improved a lot on the clunky mechanics of the first game (and were just excellent), Black Flag was in that same tradition while also managing to be one of the best pirate/sailing-combat games around. If you went straight from the first AC to Black Flag, you'd have a lot to learn to catch up. Agree with @Soul that Black Flag is a stand-out game. But if you have the time, I really think the Ezio games are worth playing.
Others in that sequence (AC III, Unity, Syndicate) were a bit meh. But with Origins it's expanded into a more traditional open world RPG. Most people like it and feel the character of AC is still there, but I do know others who feel it's just another RPG now.
Are any of the more recent Assassin's Creed games any good? I enjoyed the first one when it came out but haven't played any since.