Quake 4 is something of an oddity in greater scheme of things, part of ID's recent comeback that began with the mediocre "Return to Castle Wolfenstein" and peaked with the phenomenal Doom 3. Quake 4 almost slipped under the radar, developed outside of ID (by the excellent Raven Software) and attempting to to ignore Quake III: Arena somewhat; Quake 4 continues the story from Quake II and picks it up seamlessly.
If you've played Doom 3 then the first thing you're likely to notice is that the games are incredibly similar. The visual similarities extend to almost identical armour for the marines in both games, similar looking weapons and very similar environments. However, when the game is played further the differences become clear. Firstly, the focus of quake is not to frighten but to excite. The game is far more action based and many of the situations involve blasting away while looking for the next facility to destroy. These sections are carried by an excellent plot conveyed by the marines accompanying you. These marines all have names, ranks, roles and personalities; most significantly the techie Strauss who progresses from being a pain in the neck to a truly loveable character. The player is also given some wonderful, though frequently spolit plot twists than really absorb the player in the game.
As for gameplay, it is a strong FPS with responsive controls, a good selection of weapons and some truly excellent level design. The only drawbacks are the rare but still intrusive tank levels intended solely to compete with Halo.
Furthermore, the multiplayer on this game is incredibly fun, it does have its drawbacks. It appears to be short of maps and no bots are included. You're required to find all the players and this can be quite a struggle as the game was something of a led balloon on the 360.
If you enjoyed Doom 3, like first person shooters and games with a good balance between action and plot then this game is worth a look.