I'm cautiously optimistic. The Supremes at this point appear fairly united about the concept that the 2nd Amendment applies to an individual right to keep arms for private purposes, and the remaining questions seem to rotate around to what level of scrutiny THAT regulations of that right must be held, and secondly, to what extent the "bear arms" piece may be regulated.
At a minimum, I think D.C.'s ban is toast. If the Supremes grant one of my fonder wishes, coming down with a decision declaring that any regulation of the rights recognized by the Second Amendment must be held to the same strict scrutiny as those described in the First...why then I expect great shrieking amidst those I dislike intensely as every gun control law since the New York Sullivan Act (1911) forward is brought into question (and likely struck down).
I don't think we're going to get THAT lucky (I'm betting the Court will follow the Miranda model and go for a series of incremental decisions to arrive at the same location).
And no, I don't anticipate the United States descending into some great wild west shoot out if we do get "that lucky". I do anticipate, should we luck out, that the crime rates in Massachusetts, California, New York, New Jersey, Hawaii, Illinois, and other unfortunate areas will likely drop off something fierce - at least those crimes where a significant risk of being confronted by a proposed victim exists.
And, regardless, if the decision comes back with anything other than a conclusive denial of an individual right (something I see as rather unlikely) I anticipate watching with great glee a large amount of liberal hand-wringing and bed-wetting, followed by their looking a tad silly when the world fails to end on schedule.