Not-really spoiler: If you are going to go see G.I. Joe, do not go with anyone who knows anything about nanotechnology or particle colliders. And while the nanotech fail was within common limits for nano fail, the particle collider problems were massive. And confusing. And I was left wondering if anyone producing the movie understood what 'shielding' is.
Basically, the good bits of the film: Christopher Eccleston and a few of the early explosions, specifically the pretty ones that deconstructed helicopters.
Not so great stuff: There weren't enough hot guys in uniform, the explosions were frankly boring, the main battle sequence was... familiar (I'm not saying they directly lifted, by storyboard, from A New Hope. I'm saying there was an awful lot that reminded me of Episodes I & IV. And then they got Titanic into the act), the property destruction was dull (okay, Half Blood Prince managed better in collapsing a piece of structural engineering, and G.I. Joe got to play with THE EIFFEL TOWER), and there were definitely no animals hurt in the making of the film. They CGed the lot, and the CG was... quite obvious.
Also, families are bad things. The only thing families do is hurt one another.
Also, if you want to have a flashback, sit in a plane and stare at your hands. Works every time.
However, I'm hoping Natasha Romanoff's wig for Iron Man II turns out as well as Scarlett's did. Not that the wig was amazing, but the one they currently have for Black Widow is way dodgier looking.
The painful bits: NANOBOTS DON'T WORK THAT WAY! Except, okay, yes, in movies and cartoons. Actually, most of the painful stuff is probably a carryover from a large continuity. Like the way everyone was related to everyone else. However, nobody gets forgiven for the particle collider scene. I'm not entirely sure what it was they were trying to do, but... colliders don't pulse into unshielded rooms. They don't 'activate' nanobot warheads. They don't make things float. They don't build particle accelerators under cities. Not because it isn't safe, but because of vibrations from above, and the fact you need lots and lots of distance to accelerate along. And the facilities tend to be built of concrete and lead, not glass, due to the fact that accelerators have to be kept EXTREMELY COLD and they produce radiation all along the electromagnetic spectrum.
It was good to see a bottomless pit in both the Joe headquarters AND the Cobra headquarters, though. Nice ignorance of the Evil Overlord List.