Assassin's Creed had a great concept and setting but was held back by the repetitiveness of the gameplay. Each memory sequence was the same: get your target, go to relevant city, speak to assassin, complete the same three activities, go back to assassin, complete assassination and then return to Masyaf. At the halfway point, I did struggle with this but Altair's journey and the Crusade setting kept me playing.
I would recommend Assassin's Creed to people who have played some of the later titles, especially Assassin's Creed II, who want to see how the series began. However if you are a newcomer to the series, don't start with this game but i would recommend Assassin's Creed II instead.