Assassin's Creed (AC) is in many ways a successor to the Prince of Persia (PoP) series. It brings a sci-fi twist to the story, and perhaps a hint of modern politics, but I wouldn't say that it's more adult than PoP because of this, as in many ways, despite it's fairytale setting, PoP was quite an adult product. I mean by this that it never felt as if the developers of PoP were talking down to their audience, and reassuringly, the same is true of AC.
The basic experience of playing AC is one that tends to initial interest and excitement as the game introduces itself, followed by a period where repeating various tasks in the game begins to become tedious. However, persistence rewards the player with the skill to trivially execute the more repetitive tasks quite quickly, and they cease to represent a chore. It is only in the latter stage that the game begins to show its true form, and remains satisfyingly engaging until the end.