I love the SP so far, it's just like COD4 which funnily enough I only just completed and played for the first time recently. I've always been a huge fan of COD1 and 2, and I always thought the 'Modern Warfare' thing wouldn't be as good. I've been so, so, wrong. I'm glad I got into COD4 only a few weeks ago to appreciate this because the SP is sooo good.
I love what they've done to the gfx engine by the way, I know it's old but they've really tweaked it well. Visually it still looks impressive, although you can see in some parts where it's showing its age. The bonus of course is that it runs great on my pc and doesn't need really high spec hardware to get a playable frame rate.
It's got the usual COD gameplay, with lots of checkpoints, scripted sequences etc, but that's what COD is. There are few first person shooters out there that really give you the atmosphere that COD does. I'm glad they kept the gameplay element still there and didn't do what most games do these days, which is constantly going for the best graphics rather than making sure gameplay is half decent.
I may try out the multiplayer at some point, but I've never really been a big fan of COD's multiplayer - I find it all sorta frantic and deathmatch like, unlike the SP which is very tactical but certainly the CO-OP mode interests me.