Republic Commando uses the Unreal Tournament 2004 engine. However, without prior knowledge of that, you never would have guessed. The folks at Lucasarts have successfully transformed an engine that was originally meant to run fast paced shooters such as UT2004 into an excellent squad based shooter, adding all sorts of great elements that weren't included in the original engine.
Speaking of these elements, they really do add a lot to the game. On the easier difficulty levels, you might be able to go through the different levels running and gunning until the very end. However, once you start to raise the difficulty, the game will force you to use the squad commands to survive. Your squadmate AI is fantastic, and unlike in some other games, they'll actually do what they are told.
The single-player is sort of like a three pronged attack. It has action packed gameplay, great squad based elements, and it also does a great job of maintaining a Star Wars feel. This is even more impressive, considering that the Republic Commandos were never really explored in the movies or in the Expanded Universe prior to the game's release.
The production values are excellent. The graphics were excellent for their time and Republic Commando's music and sound is top notch. The single-player levels are all very entertaining, the levels, while not as varied as you may expect them to be all have their own unique feel. Especially with their objectives. The story rolls along nicely, and it's a really difficult game to put down until you've beaten it. However, some of the battles that take place in Republic Commando seem to lag on forever. If there is one problem with the single-player, it's definitely this. Most of the battles involve super battle droids coming out of dispensers that drop from space. They'll keep on spawning unless you can destroy the droid dispenser. It's pretty important to destroy it quickly too. The super battle droids can take a ton of hits, and after a while, it starts getting really old. However, this shortcoming is made up for, the boss battles are excellent.
Despite a great single-player, Republic Commando can't really be considered an AAA title and here's why. The multi-player. It's that simple. It's just terrible. The multi-player maps are all pretty uninteresting and there are some serious weapon balancing issues. The selection of modes is pretty standard. You have your deathmatches and your team deathmatches. You also have a basic Capture the Flag type mode. There is also a game type known as Assault. Assault is fun for about five minutes until you realize how unbalanced the maps are for one team or another.
In general, I'd avoid the multi-player, if you're looking for a Star Wars game with excellent multi-player, I would recommend Star Wars Battlefront 2 or Star Wars Jedi Knight Jedi Academy
All in all, Republic Commando is a solid title. If you see it in stores, I'd recommend picking it up. The great single-player is worth the price of admission.