Oh I've always enjoyed the singleplayer in those games, just like I did enjoy this one too... but I don't think they're good enough to justify the price. Not by themselves. The Modern Warfare series in particular (along with the Black Ops side-step) is one with good singleplayer, decent storytelling, great spectacles and so on... but it's just not long enough nor does it have the replayability required for me to consider it worth the money by itself. Less than five hours of gameplay? What the shit? Sure, those four point something hours are good enough fun but come on. I can buy a huge stack of movies for the same price and get damn close to the same level of entertainment over a much longer period of time. Especially since the games are so enormously linear that replaying it is pretty much just like watching a movie again. You know exactly what'll happen and when. You don't even have the RPG element where you can change the gameplay through changing your playstyle/character.
I expect more than four point something hours of perfectly linear gameplay when I dish out fifty-sixty bucks and that is where Multiplayer comes in. The Battlefield series (and after having played BF3 multiplayer for a bit) rocks my world in that regard. That is where it rises so far above the CoD/MW series that it's practically in geostationary orbit over a gasping asthmatic snail.
EDIT: To add to the above: I can actually see spending sixty bucks on a four point something hour long singleplayer campaign without multiplayer attached if it actually blows my mind a little. BF3 and MW3 does not, because it doesn't innovate. Where Portal and Portal 2 are short and sweet little games, they still manage to cram in something innovative and new, changing either the gameplay or the story/plot enough to throw me a curveball. That just doesn't happen in MW3 or BF3's singleplayer. Until they do, Multiplayer is what needs to lift it up to make it worth the money.
Revised recommendation: If you want fantastic multiplayer with a decent enough singleplayer on the side? Buy BF3. If you want good (but very short) singleplayer with a meh enough multiplayer on the side? Buy MW3 once it's on Steam's Christmas sale.