Assassin's Creed was on of the most anticipated games to be ported to PCs. For this, the Director's Cut edition was produced, adding some content over the console versions (mostly rooftop action missions), and care was taken to make the gamepad to keyboard/mouse-transition as seamless as possible. For the most part it was successful.
This is a tremendously beautiful game. The game is not new yet the first thing that grabs you is how real the city environments feel. The graphics are just out of this world! If your PC is a few years old, it has to be least a mid-range to enjoy their full potential (minimum requirements provided below), but real skies, dynamic shadows, facial expressions and realistically flowing robes are only beginning to describe it! Run on a roof and the other citizens will gather around and comment on your crazy behavior! Throw someone on a vendor's cart and he will come after you complaining about his ruined produce! And the city is alive well beyond your character. If only Bioware could take some lessons for its next Baldur's Gate...
Adding to this is the wonderful sound! From the crowd murmurs and the NTCs cries for help, to the whistling of the wind and the well-chosen background music, a good sound-card and speakers set is recommended to truly enjoy this game. If you have a 5.1 speaker system (I do not) I can imagine the experience to become even more immersing.
As to the gameplay, you control Altaïr ibn La-Ahad ("The Flying One, Son of None"). He is a member of the Assassin Brotherhood that sides, well, with both...sides, during the 3rd Crusade. In a story twist, he is also your ancestor, the game being your/his flashback memories. This is a twist I could do without, but I would guess it lays groundwork for the sequels.
Controlling your character with a keyboard/mouse takes a lot of getting used to as you have to manage running, climbing, fighting as well as modifying your actions from low to high visibility. The keys are re-mapable but the complexity of the controls will never let you forget you are playing a game.
The Third person perspective works beautifully and will never loose your interest. Most missions require sneaking and murdering in the shadows. Others will have you eavesdropping for passwords or pickpocketing documents to gain access into target buildings. Some will have you sharpen those sword skills. Still, the game does not avoid its share of stupid "keep this...suicidal character from getting killed" missions. Keep in mind though that Assassin's Creed is rather a strategically thinking action TPS, not a hack&slash fast-paced one.
Pull your knives out and sky-dive into it!