Well paced, gorgeously filmed, mystery-action-adventure with a deft, comic touch. Downey is a brooding, physical Holmes (think Daniel Craig's Bond), but also a selfish, petulant nerd (think of Gregory House). The plot is silly, but that's ok: this is just for amusement, and never drags.
Nothing to shock the kids, especially tweens and up. Younger kids would likely need too much explanation.
One leaves the theater hoping for a sequel (or prequel).