Republic Commando is as good to play today as it was in 2005, and you can't say that about a whole lot of games. Its been pretty timeless for a while now.
The special thing about the game design is how involved you feel. The way the screen (by default) has the helmet graphic to make you feel as if you are in the suit. The way the other commandos engage in banter as you go along, so there is that bonding with them throughout the story line. It all makes you very attached, and makes Sev's abandonment, for me at least, genuinely annoying. The lack of sequel is a big disappointment; arguably, the game has an ending that is too abrupt. It is such a painfully massive cliff hanger that cancelling the second version is extremely frustrating.