Just seen it today. Alas, the standout moments for me are a bit too spoilerish to be revealed right now, but suffice it to say...oh fuckin hell, it's great. One thing they got right was how each of the superheroes were introduced into the movie; nothing felt contrived or shoehorned in - there was a distinct impression that they were all brought together for reasons that went back into their own individual storylines. In fact, a number of plot elements from the previous standalone superhero movies had resonances with this story, and it worked really, really well.
I saw the 3D version - I'd say the effect was workmanlike but not standout; then again, the 3D did in no way distract from anything that was going on on-screen. So IMO you're not gaining or losing much from watching either the 2D or 3D version - it's just down to a matter of personal preference.