If you let your kids watch Iron Man, then you should be ok with this.
While I agree this movie has tones of violent action, there is little blood and most of the violence is over the top and cartoon-ish.
The decapitations are, for the most part implied, and never actually seen (no rolling heads, and no extended or clear shots of the bodies). These happen so fast my son didn't even notice them.
My two biggest concerns that parents might have are with the very begining...a man has a hot iron mask put on him and while we only hear him scream and see his eye wide with terror, it could be scary for smaller kids.
Another is another scene where a dead mans head sort of melts. It's brief, but again could bother younger kids.
Over all this is a fast paced film that will keep most 9+ engaged for the the very quick 2 hours this movie runs.
If you had no problem with Iron Man or Transformers 2 (G.I. Joe doesn't even come close to the languange Transformers 2 used btw), then you won't have a problem with this. Totally fun ride!