Considering that Quake 3 opted out of a singleplayer campaign entirely, I wasn't expecting much from the Quake 4 singleplayer campaign. But it wasn't half bad. It's a title you'd want to rent, not buy, but at least I was entertained the whole way through.
The aspect that keeps Quake interesting is its variety of fun weapons. I've been on a FPS singleplayer kick lately, and I've gotta say that the weapon selection in the games out there is boring me to tears. I don't care if the rate of fire is different and the clip size is larger - it's still just another freaking machine gun! Quake 4 has a machine gun, but it only has one. And it's got a lot of other weapons that are, you know, fun.
Killing someone with lightning is fun. Creating a dark matter vortex is fun. Gleefully lobbing grenades everywhere is fun. Shooting hundreds of tiny exploding nails is fun.
Well, it's not all fun. Getting stuck with a crappy save where you have 25 health is no fun. I'm not sure why in this day and age shooters are still struggling with health/armor systems. Halo solved this problem years ago: give the player regenerative shields. As long as you survive an encounter, you're reset to a known-good state of health. It makes game balancing way easier, and less obvious for that matter ("Hey, here's a big unattended cache of health packs and armor! I wonder if there's a boss encounter coming up..."). The downside of course is that with no health system at all (as Halo 2 opted to do) you lose the sense of being "worn down" over some long epic encounter. I'm not saying ditch health entirely - I'm saying get with the times and start thinking about "out of combat" health resets/regeneration. And obviously "shields" don't fit into every shooter's lore, but the same idea can be applied anywhere. Hell, if you're already in the business that saying that health packs can heal the protagonist's bullet-riddled body, you should be flexible on spontaneous regeneration.
But ignore the antiquated health system and long load times (which you may get very familiar with due to said health system). I applaud Quake 4 for not trying to be too serious. Sure the whole space marine motif is almost as tired as WW2 shooters. But at least Quake 4 doesn't waste effort trying to make too much sense. These are games people, not combat simulators.
The one inexcusable aspect of Quake 4? The frame rate. This is a console, not some random PC with a bargain bin video card. Get with the program.