A much better version of Doom 3; Quake 4 rises to new levels in gaming despite its painful linearity.
Although the game is very linear, the AI is simply great. What with all the added improvements, Quake 4 is certainly in competition for one of the top 5 games of the year. The sound and physics engine were used to full capacity and made full use of its potential.
There are a lot of similarities between Quake 4 and Doom 3, especially when it comes to introducing new monsters; level designs and such; however, the differences create another dimension of improvements over Doom 3.
My only other Qualm with the game was despite all its intentions, it was a fairly easy game. I played it on the second hardest setting and it still wasn't much of a challenge. That being said, I really liked it a lot and I think most people will agree that for what it is, it's really good.