PC high-end graphics card = $600. Xbox 360 Pro + Quake 4 = $460...tough decision?
For starters, the graphics in this version (from here on I'm only going to speak about the Xbox 360 version) are very good. Environments are crisp, character models are nice, and even the weapons look detailed. Is it the sweetest looking game on 360? Not necessarily. BUT, it's no ugly duckling either. I went into this game expecting a Doom-esque gameplay experience, and I got that but with a TON more action...which is good, considering Doom 3 was one of my biggest let-downs last gen. There's alot of gore in this game, and I feel ID and Raven did a good job to draw the gamer into this Quake world and make them feel like a part of the action without going overboard on effects like blood, etc. I did note that during the opening cutscene, the framerate jogs a bit (no more than Perfect Dark Zero during in-game cutscenes) and there are points in the game where it does slow considerably (always when there's a whole lot going on at once), BUT I never felt it to be a major hinderance. As before stated, feel free to throw some serious cash into your PC so that you can run this one game without the framerate issues, but for me, it's simply not worth it.
The sound is good, although most of what you'll be hearing is weapon's fire. And that is a good thing too. This game will throw waves of enemies at you, but I never got tired of blasting away with the shotgun. The one audio issue to mention is that when your allies speak to you in game, you can almost never hear wtf they are saying. Fortunately, there's a box that appears in the top left corner to tell you your objective anyway and hearing the words isn't necessary...but it does get a tad annoying.
Control is good. Sometimes you may feel like it's taking you all dang day to reload your weapon, but movement is never really an issue. Late in the game, there are some parts that require jumping. (FPS jumping...can we please just get rid of that all together like PDZ or at least not make your survival count on you making a specific jump or series of jumps?) That's a little tricky but after a few tries, you get by and move on.
Entertainment....well, truth is, once you beat it, you're probably done with it. Unless you love some seriously fast-paced multiplayer that plays as if everyone's stuck on fast forward. I'm a jaded FPS vet, but the tempo is just a little too over the top for me. I haven't played through single player since I beat it, and I feel the need to mention that the final boss of this game is incredibly anti-climatic. I won't ruin anything, but let's just say...you'll get it somewhere between one and three tries.
All in all, Quake 4 is a good game to play through at least once to see the excellent story unfold. I wasn't completely happy with everything they did (you become a Strogg [no spoiler...everyone knows it]), but it was a fun time while it lasted. And unless you've got money to burn, checking it out on 360 will be your only option for now. I recommend a rental for sure and if you like it, a purchase.