I bought CoD4 not to long ago and I'm loving it. The online is really deep, tons of maps and modes to choose from, and it's easy to find people playing as there are around 100-120k people on at a given time.
Yeah, definitely. I got this like a year ago and I still play it very frequently. It just never gets boring for me, and I do like it more than World at War. (Except for Nazi Zombies, which is awesome).
I really cannot wait for Modern Warfare 2. I hope they put in a similar mode to Zombies, trophies, and split-screen online (fat chance) and my wish list will be complete.