After listening to excited Future Publishing staff rave about this all day, I downloaded the CoD 4 multiplayer beta this evening and managed to get in a good hour of blasting. It's a perfectly tuned multiplayer FPS experience. Nothing massively unusual yet - the classes (sniper, infantry, heavy weapons, spec-ops etc) are familiar - but I've not even begun to explore the RPG-esque character customisation system that allows you to unlock new weapons and skills as you rank up.
Game options are familiar too - the usual team and every-man-for-himself options abound. The lobby service is cut down to the basics - select your game type and the hunt for servers begins (you can also set up a private match). I was on and playing within a few seconds.
Everywhere, in every element, the implementation is so slick. The maps I've seen take in burned out villages and battered industrial complexes, all riddled with passageways, sniper points and blind alleys. Apparently, you can shoot through walls (depending on the bullet-type and integral strength of the barrier), making those long camping sessions behind paper-thin plasterboard a thing of the past.
Action is incredibly fast and frenetic - one or two shots and you're dead (then you're treated to a quick sequence showing the kill from the shooter's perspective). It feels like the tactics and severity of GRAW mixed in with the twitchy immediacy of Counter-Strike. Several sites have been eulogising the C4 system - basically plant it and doubletap 'fire', rather than having to use a separate remote control mechanism. There are nice little features, too, like the ability to earn radar time for airstrikes by getting three kills in a row without being fragged yourself.
Just an exhilarating, super-smooth online experience.
UPDATE: Just to make things a bit clearer, the beta is running on Xbox 360. In the UK, Gamespot is offering codes, but I suspect they're going to be over-subscribed. Check the official site for more info.