Republic Commando, rather than trying to encompass the entire story line of Star Wars, follows the story of Delta Squad, a group of 4 Clone Commandos fighting for the Republic in the Clone Wars. In terms of game mechanics, it is in many ways similar to your average FPS, although the addition of squad AI elements are what really make it stand out. I'll try to break down the game as best as possible into subsections in this brief review.
In the campaign mode, players relive the Clone Wars from the perspective of RC-1138 "Boss," the leader of Delta squad. The beginning of the game is essentially an in-level tutorial, starting the player off solo and gradually introducing them to the major mechanics of the game, most importantly that of squad AI command. It's not difficult for anyone even vaguely familiar with FPSs to pick up quickly, and the AI elements are particularly interesting.
In short, the player is backed up by the remainder of Delta squad, Commandos Fixer, Scorch and Sev. Each other commando has his own "personality," from comm chatter to prefered weapons and skill specialties. The player is able to issue squad members various commands, such as defending certain locations, reviving fallen teammates, slicing doors, assuming sniping positions, etc. This is perhaps the most unique and, well, fun components of Campaign mode, as the AI is remarkably well-done. As for the campaign itself, the game does a good job of getting the player into the action quickly, and keeping the story interesting, well-paced, and, at times, suspenseful.
Now, I had two main complaints regarding the campaign. First, the story seemed to be somewhat "railed" at times, requiring the player to stay on the path to advance the plot and not allowing for much exploration. Then again, this game excels more action-wise than option-wise. Secondly, I just found the story to be too short. I would have liked to simply see more content added, even if it was in the form of side-missions that don't necessarily contribute as much to the overall story arc (or just slow the main story down slightly.) Continuity-wise, there are year-long lapses between chapters in the game, which clearly would have allowed more content.
The real "staying power" of Republic Commando can be attributed mainly to its multiplayer mode. I'll admit up front, there isn't a whole lot in the way of innovations in RC multiplayer, but the experience is well-done enough that there are still countless servers running 7 years later. Multiplayer includes the staple gamemodes: deathmatch (team and solo) and CTF on an impressive number of maps. Weaponry is discoverable across the maps rather than unlockable, and the range of weaponry ranges the gamut from sonic grenades and pistols to concussion launchers, sniper rifles and miniguns. It might be worth noting that anti-cheat generally works well (but server Admins are capable of enabling certain cheats such as low-grav if wanted.) Overall, multiplayer brings the atmosphere of the Campaign to large matches well, and has decent replay value.
Overall, Republic Commando is still a game worth playing over 7 years after its release, which is saying something considering its short storyline. Graphics are great by 2005 standards, and still look good without stressing any decent system to run on max. Both single- and multiplayer successfully capture the "spec ops" atmosphere of the story, and the squad AI system is unmatched by any game I've played since. This is definately a game worth checking out, either for the campaign experience or multiplayer combat.