I thought all the Dark Forces/Jedi Knight games were great fun, but that's just me.
I enjoyed Forces of Corruption. It has flaws, as pointed out previously, but overall I think it's pretty good. Plus, seeing the SSD in a fight is just jaw-dropping.
My biggest complaints that have not been said thus far are as follows: When the Executor is in system, it seems to have a bug with it where all of the enemy's ships have basically unlimited range and extremely rapid fire on all of their weapons, no matter who they're attacking. I lost a Star Destroyer to a quartet of A-wing squadrons this way. They'd start their attack run, and the moment they start, they'd be able to begin firing. They would not stop firing until they pulled out of the attack run. So I'd have a stream of lasers from an A-Wing, literally miles long, all hitting my Star Destroyer. So I got to watch the shields melt away, and guess what the first hardpoint to fall was? You guessed it, the shield generator. So now I have a completely vulnrable ISD and four enemy A-wing with ultra-superpowered weapons. The next battle I fought this way, I won, for no greater reason than because the SSD has about a billion hardpoints, but I think I lost 5 ISDs, 5 or 6 VSDs, Piett (with the emperor), Thrawn, and a few smaller ships, while the enemy had about 20 A-wings, 30 MCCv(I think? the long torpedo frigate things), and 2 Mon Calamari cruisers total.
Another thing I hated was the way corruption treated in-orbit assets. For example, if I had a level 5 space station around Kuat, and Kuat was corrupted, the enemy could have a starviper squadron in orbit around Kuat with no battle. Thinking it's only a single fighter squadron, I could easily beat it. So I throw a single Victory Destroyer there. Guess what, even though the planet is mine, the level 5 space station has become my enemy's. Not wanting to lose my destroyer, I retreat. Well, now there's no more space station around the planet. There goes a ton of credits and a whole lot of time.
I do love the balance between the three forces, though. I have such a hard time gaining ANY ground whatsoever, it's probably the best 3-way strategically balanced game I've played in a long time. It can be frustrating sometimes, but I enjoy it. You waste all your money removing corruption, or you end up rebuilding your facilities. Then you have no cash left for your fleets. If Zann's forces are able to land, you've pretty much lost the planet, but then, you can destroy the planet with your Death Star. Although, that leads to less money. Etc.