HomeLan Fed posted very interesting news-article with their impressions and description of what Doom III Multiplayer QuakeCon demo is like, here is the summary it self:

The gameplay itself was standard deathmatch; in other words running around in a level going after the three other opponents and making sure they are dead. There was nothing terribly innovative, but it was still fun.

The partial list of weapons in our demo consisted of the pistol (very weak), shotgun, (strong at close range), machine gun (very effective), plasma gun (shooting white balls of energy) and the rocket launcher (no explanation necessary there). You can also use your fists to attack but they are really only effective when you have the Beserker power up (more on that later) and you also have a flashlight for seeing in dark spaces that can ales be used as a melee weapon. As you might expect, ammo for the various weapons in the Doom III DM demo were all over the map as was armor.

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PlanetQuake3.net also has min-review:

The Doom3 Multiplayer computers are inside a booth that looks very similar to the one id Software showed Doom3 in last year. I walked inside the room and they first told me and the two other people playing with me to set our controls. Once that was done, I was asked to start the Doom3 server by clicking on Multiplayer, then Start Server. Then the two other people connected to my computer and the game started. The speed of Doom3 is very reasonable I think. It's not as fast as Quake3 and strafing is no where as good as Quake3. Some of the weapons in the game include fists, a flashlight which you can use to hit people, a shotgun similar to the one in Quake2, and a rocket launcher. The three different players were listed as different color.

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update №1: We got some new info on Doom 3: