While the campaign on this game is somewhat mediocre for a first person shooter, most people picking up Battlefield 3 are going to be interested in the online mode. In comparison, the multiplayer should be the selling point of this game - forget the campaign. Many want to compare this game to Call of Duty, and perhaps that's valid. As a basic comparison, Battlefield provides a somewhat more realistic and (dare I say) adult online environment compared to it's competition - don't expect to find a large community of snipers doing 'trick shots'.
Gameplay is far more team-based than most other titles and this works really well. If you're looking for something which resembles a typical deathmatch-esque shooter, this probably isn't the title for you - the Battlefield series isn't about running around alone shooting everything - there's much more strategy involved and playing a specific role is a must.
For a few quick points on the multiplayer:
+ Great selection of weapons which are largely well balanced.
+ The new engine looks great and functions well. Few glitches.
+ Getting into a game is surprisingly easy, I've had no issues with matchmaking.
+ Vehicles are very well implemented, adding a great deal of depth to the game.
+ Unlocks, unlocks, unlocks. Pretty much everything can be modified. However, it would be nice to have some more starting unlocks for vehicles, since you may struggle at first with limited aircraft weaponry.
+ Very good maps - beautifully sculpted, although destruction tends to change that pretty quickly!
- Some bugs do exist. Few are a problem, and should be patched soon. I'm yet to find anyone exploiting them.
- Menus are somewhat clunky (not terrible given the sheer amount of customisation). This is irrelevant once you're in a game, and so shouldn't stop you from buying it.
- Needs a mode for flight training. Would advise against jumping into air vehicles as a passenger for the first few weeks, since crashes are common for new players. Flying isn't too easy or too difficult, but does take some practice, which can be very frustrating for your teammates.
It's worth noting that patches are quite large in filesize, but do make a LOT of improvements. There's some indication they may revamp the UI to one designed by the community - DICE do listen to their customers.
Overall: It's a good game, with enough features to keep players busy for a LONG time. COD players should give this a go if they're looking for something a little different, since the two play very differently. Some bugs could use work, but with any luck DICE and EA will pull that together. With DLC coming as soon as December, it's looking like a bright future for this game.