Meh, the game is better than it seemed initially, largely because the opening few levels are truly pathetic, both terrible story, terrible level design, terrible everything. There are "stealth" bits where you walk right by the enemy and they can't see you, weird mechanics holding up an injured guy, terrible boating bit, etc, etc. It improves dramatically, but still goes from absolutely terrible to incredibly average.
I'll give it some credit, having some real choice on weapons for most missions is sorely missing in most games, however, its broken, there is more than one mission you choose your weapon, only to start the level with different weapons anyway making it rather a joke. Some of the things you try, certain sights, certain mods, are useless and very very boring to play with and make guns horrible and there isn't an easy way to pick up a "good" gun for an entire mission after that. Its painfully easy so picking up a gun without a good sight barely makes a difference.
The mechanics of the tactical levels is crap, the idea is potentially ok, its NOT new despite IGN insisting its a massive change. We've had levels with the zooming in and out and directing guys from above, this is far far more dynamic, but ultimately broken. Each and every mission the AI is terrible and 99% of the kills end up made using the same unit playing in person and its like playing a multiplayer mission with bad bots and terrible controls. I don't mind they included it, it breaks up gameplay and makes it feel different, its just not done very well.
Almost all driving, plane, boat, tactical missions are very poorly put together and aren't very fun.
Story, the story was absurd start to finish, and as much as again IGN(were paid) to say the story was great and new and that somehow you don't know what to feel about the main guy you're fighting, lol.
There is something wrong with the story when EVERY single level, every single cut scene you go "wtf is this, no one would ever fall for that".
Overall, the first few missions give an incredibly poor view of the game, its much improved but still not anywhere near great after that. It's not worth over £20, its worth a play through, its well worth sticking out the first few missions. It could be, as a SP game, much much worse, it could be much much much better though.