Jack the Giant Slayer is not great, but not at all bad either. Go in with the right mindset, and you'll have a good time. One note on the 3D: though one or two shots in the movie look really cool with it, most of the movie doesn't use it at all. You won't miss anything by saving yourself some money and seeing it in 2D.
Will you see Jack the Giant Slayer? Let us know what you think!