The Force Unleashed does a good job of linking part 3 & 4: with the Dark side secretly plotting the Jedi's demise.
The Star Wars franchise needed to start moving in a different direction, too many games had just exploited fans of the films, and not given much back at all. The Force Unleashed takes on a new unseen character; Starkiller. Darth Vader himself takes Starkiller under his wing and trains him, in the art of the dark side. The story flows well, although cut scenes are a bit sketchy for a big block busting game. Lots of Force upgrades keep intrest fresh, but some battles seem a tad repetitive.
Bringing down the star destroyer should have been left out completely if you ask me, but hey ho. At the end, when your light sabres last spark has been burnt, and its all done and dusted, there's a nice sense of achievement. But far from being put back in its box and left on the shelf for dust to gather, your find the game back on again, so you can see the the final upgrades at work, whether it being force blasting Jawa's through windows, or pulling Tie fighters of production rails, there's still things to be done. Various costumes, intriguing Jedi's you wont have seen before, Rancor monsters, its all worth a second crack at it, because by this time, you'll be the most powerful Jedi Master in the Star Wars universe.