(actually my first post on this forum... strange, I've been following the mod since 2007, when UT3 wasn't even released)
Anyway,
The Good:
Obviously the multiplayer experience. For some reasons the Renegade gameplay has (almost) always stayed unique. It demands teamplay among players, rewards defenders/engineers and is actually quite balanced despite the different characters.
The Bad:
The bugs and the possibilities for cheaters. The terrible support (I'm still waiting for a serial for my server after 4 years!). The cancellation (twice!) of Renegade 2. Also the single player was disappointing and since gamers ussually start off by playing the single player, I feel that many never really got to the multiplayer and thus never found out about the awesome online experience.
The Ugly:
Well, the graphics. At the time of it's release Renegade already had sub-standard graphics compared to it's peers. I personally didn't care that much about the graphics... being an older gamer (since the 80s) I know that gameplay is a 1000 times more important. I also always used the excuse: "Well, it's the first time Westwood builds a FPS-engine, they'll get better with Renegade 2."
The main problem: Weak graphics and singleplayer made many gamers disappointed and decide not to try the game any further. A missed oppertunity. (at least that's what I think is the reason for it's disappointing succes)
As for me, I started playing with the online beta/trial, so I found out about the multiplayer before the singleplayer. Renegade is the only game I really had fun with online. I love Unreal Tournament ('99, 2004 and 3) but I still play it offline. Being a real team-game gave me the chance to really get somewhere (in the erea of not-being-a-noob) without being intimidated by pro-gamers.
C&C Renegade is probably my most played C&C game, despite the fact that I went through several Red Alert discs over the years.
Still my favorite online experience!