Probably one of the best designed games ever, and has set a crazy standard for everything that comes after it. You definitely want to take a good week out of your life to do nothing but play through the game, but that'll probably only get you through the main storyline. If you really want to experience all the side missions, you'll probably need two weeks.
Storyline is compelling, if not a little overembellished in parts, and is acted perfectly. You'll want to see it through to the bitter end (and then some).
The Online component is not as impressive, but they've continued to add to it!