COD4 is pure greatness!  the single player campaign is one of the best but
I'll focus on the multiplayer, which is also better than Halo in many ways.
And I've been a die hard Halo fan since the beginning (from the days of
Marathon).  COD4 is really the best right now.  It's won top spot for best
online game of the year.

I will warn noobs that it "feels" different at first but you have to move
through this.  It is not Halo.  The first few ranks are really for
"acclimation" but you will also rank up quickly and unlock new weapons
without too much frustration.  The feel of this title is more "realistic"
action.  You also move faster than in Halo, which is turtle speed.  You can
sprint, prone, jump, crawl, etc., and malee' is super fast and you use a
knife.  The aiming system was different at first to me but has proven itself
to be better.  I love the Class creation.  The ranking system is so
preferred that Rainbow Six Vegas 2 is rumored to have based theirs on the
same model.

Can't forget about the graphics.  Absolutely the best, better than Gears (no
offense to Gears die hards).  It's very realistic.  The war torn towns
strewn about with rubble, rebar, debris, destroyed buildings and vehicles,
beautiful environments... the blood that sprays from the targets when
they're shot and how they fall and quiver...  You can call in Helicopters
and fighter jet airstrikes...  the battles are intense and multiplayer
offers more than a dozen game types.  I've been through all 55 levels
(ranks) and I'm half way through my second time.  It's set up to reward or
track your ranks for 10 times through.

A new map pack is due in Spring.  I wouldn't expect the price of COD4 to
drop anytime soon.  It's top ranked and hundreds of thousands of players are
on at any given time of the day.

It's worth it's price!  Do not hesitate