I won't go too much into the story because... I didn't feel like it had much of one. You're a force-sensitive child that Vader picked up and trained as his own personal assassin, which I thought was an idea that kind of works. TCW (which was still co-existing with the EU) had Anakin take up Ahsoka in the movie and show, so him having a new Padawan (or Sith Apprentice, in this case) makes logical sense to me, like he's trying to fill a hole from her. At least, that's the vibes I got; I know that it came out the same time as the TCW movie, so it might've just been a coincidence, but that's my headcanon, helps to me that Starkiller and Ahsoka also both hold their lightsabers backwards.
As for the main plot: I've heard this game referred to as just a power fantasy for teens, and it does kind of come off to me that way, but in a good way for the gameplay, which I'll get to in a sec. Starkiller just didn't have much of an interesting personality to me, and his character design was too similar to Cole McGrath from inFamous and damn near every other white guy protagonist in the late 2000s (mostly because of the shaved head), and the Light Side ending just felt... okay to me, but the Dark Side ending seemed more fitting to me personally.
The idea of Vader and Palpatine using him to accidentally create the Rebels is an idea I don't necessarily hate; after all, incompetence is one of the hallmarks of fascism, and it would speak to how empires always fall due to their own hubris, but it plays against the idea that hope will always succeed on its own (though you could argue Starkiller was responsible for holding the hope and keeping it alive), and it also directly contradicts the Radio Dramas, which show that Bail Organa was at least tolerated by the Empire, but was almost executed directly by Palpatine, who knew for a fact he was leading the Rebel Alliance. That contradiction, to me, makes me wish the game was its own thing and just stuck to having the Dark Side ending as a deviation from the main series. It would have been a hell of a shock ending that stuck with you, fits more with the shit you do throughout the game by being able to kill literally anyone and everything, and it wouldn't matter what you contradict. If anything, being a power fantasy that ends with you falling into being Palpatine's puppet perfectly encapsulates being a Sith in general: Giving into the Dark Side and causing chaos is good and fun at first, but it ends up coming back to haunt you in the end. You could do more sequels this way without worrying about the canon, or at least tweak the main story by making him a Dark Jedi in service of Palpatine, like Kam Solusar before Luke turned him back in Dark Empire II.
The gameplay I found mostly fun, but with a few caveats: I do love being able to use the Force to be able to kill basically anyone and everything, living up to "The Force Unleashed" name, but some enemies and gameplay mechanics I found a little off. I'm biased in the sense that I kind of suck at hack-and-slash games; I've played the old God of War and some Devil May Cry games, so I'm decent at pulling off some combos, but I found the combos here a little hard to pull off, and didn't feel very rewarding, and it doesn't help that enemies like the Purge Troopers or AT-STs and the like knock you over from the first strike you make and can't keep comboing them. I mainly said "fuck it" and hit them with Force Lightning until they died, or it gave me the quick-time reactions to kill them.
Ironically, because a lot of enemies later are like this, I found the boss fights to be even easier and more fun than most of the levels themselves; I actually got to make better use of the combos and have a real lightsaber fight. Otherwise, the levels I go through by just picking up Stormtroopers (and occasionally those damn Purge Troopers when they're close enough to a regular Stormtrooper) and tossing them around more than I use my lightsaber. It cheapens it a bit for me in that I don't use the actual weapon of a Jedi or Sith, but I still found that insanely fun and cartoonishly hilarious, plus it leveled me up faster by doing it. I even found myself being more sadistic with it by just holding Stormtroopers in the air, hearing them scream and beg for mercy, only to casually drop them off a cliff or walk them into a disintegration wall. Again, more reason to make the ending just have you become a full Sith.
This is also me being picky, but I would've also preferred if they went ham with the lightsaber combat and let you dismember enemies, going full M-rated. I know it's a Star Wars game and that's not likely to happen, especially not now, but it would've been fun for sure, and, again, actually fit into the feel of you falling into the Dark Side of the Force and made for a fun mechanic that you don't see in any other game.
Overall, pretty fun, and I'm glad I got the Ultimate Sith Edition for $30 so I could enjoy all the bonuses. I might check out the second one sometime down the road when I get it.