The single player is better than Halo 3's. It's incredibly intense and has some unique and welcome cinematic moments.
Multiplayer, while fairly fun, doesn't stack up to Halo 3's. Because of the plethora of modes and maps, not to mention the forge and theater, Halo is still the multiplayer king. I bought CoD4 (for my roommate's 360) but I would just recommend a rent. It is ok to go back through the game, but you won't really feel the need to unless you paid $60 for it. The harder difficulties are frustrating more than anything, and like every other CoD game you don't actually need to kill anyone you're not supposed to if you're good at running and hiding. So long as you gain ground, your invincible teammates will do the rest.
Like someone earlier said, it is a very linear game, which has always bothered me about the series. Luckily, doing what you're supposed to do is very enjoyable, but a rent would suffice in giving you that experience.