Is it worth finishing Assassin's Creed 1 before playing 2?

Solution 1:

Although you can play and enjoy it, AC2 is a direct sequel to AC1, so I'd suggest you play the first one to fully grasp the whole story.
The final part of the game is very entertaining and made it worth for me to play thorugh the repetitive parts.

AC3 will be the final part in the trilogy, completing it like one big game.

Solution 2:

If you don't want to invest that much time in AC1, I don't think you'd miss too much if you just read the plot summary on Wikipedia and watch the ending on YouTube. AC2 was an improvement over the original in pretty much every aspect (particularly storytelling) and I don't think you'd be lost.