I've been playing it for the past week or so. So far, the single player is pretty mediocre. The level designs are not very intriguiing, and whenever you're just starting to get into it, the level ends, or there's a cutscene, or dialogue between your squadmates. The squadmates rarely add much to the combat, mostly just ruining that great sense of fending for yourself typical in id shooters. The enemies aren't that fun to kill; they aren't smart enough to take cover or flank you, but most of the time, they don't even rush you very effectively either. Mostly they just stand in place and shoot at you. But, I hate how when the Gunners rush you, they have so much health it's pretty much impossible to kill them before they hit you unless you see them coming 50 yards away (4 rails/rocket/nades to kill)...they run so fast, backpedaling doesn't work, and even if you manage to dodge out of the way when they swing their blade, it hits you even if you're visibly out of it's reach.

The environments do get slightly more interesting later in the game, but nothing spectacular. The vehicle segments are also really silly. The areas you travel in are so contricted, and the weapons you have are so weak, that you don't really get the sense of power that using a vehicle should. It's just a crappy diversion from normal FPSing.

The multiplayer is pretty fun though. It's a weak effort, with inferior number of maps, player counts, modes, and system performance, and almost no new features to seperate it from Q3A...but, it's fun anyway because:

1) People play it, and unlike Q3, it's not all uber-clan-tourney vets, but it's not all n00bs either. Good middle-ground of opponents.
2) The graphical update, while relatively minor, makes it a bit more interesting than staring the same shit you've been looking at for 7 years.
3) There aren't a ton of maps, but generally the ones that are there are pretty good, and not as hit-or-miss as Q3's larger selection.
4) Most of the minor gameplay changes are nice; like the nailgun, better grenade launcher, visible body damage, lighting (sometimes people's shadows do give them away), Team Arena powerups, etc. The only thing I'm not really crazy about is the DMG (Dark Matter Gun), which is not as bad as the Q3 BFG, but not as good as the Q2 BFG either. Luckily, it's only in a couple maps.

So...I don't know, it's not  a bad game if you've got the system requirements for it, but I wouldn't buy a whole new system just for Q4, expecting it to be amazing. If you're going to upgrade for other games anyway, I'd recomend picking Q4 as well, but don't expect perfection. If you want to hear more about it, I've been posting commend on my 1up blog, if you really want to hear more of my ranting; http://gnam.1up.com/ .