Have played it for a few hours in single player mode only as my new gaff doesnt have internet for another couple of weeks and have to say im not impressed by it so far either, i think MW2 plays better but TBH single player mode is crap on both of them. It just feels like an old arcade game but with a cheat mode with the unlimited lives thing, maybe its just how i play it but i dont think its all that at all.
Hopefully it'll be better when i can get on Live with it.
Got mine from Sainsbury as well - spend £30 instore and its only £27, bargain, esp as i had over £20 in nectar points :)
If live turns out to be rubbish as well tho then i may well end up trading it in, thats more dependent on how many of my mates play it tho than anything