If staying the course with sanctions will make Iran stop building nuclear weapons, why did we have to invade Iraq to stop their "WMD program"? As usual, Republicans are making little sense in their stampede to oppose everything Obama.
The truth is, once you realize that neither the US nor Europe has any appetite for an invasion and prolonged occupation, then it's obvious that developing nuclear weapons is how you force us to negotiate. As much as the Republicans want to pretend, there is no option of continuing to wait for the sanctions to work. Why would the Iranian regime suicidally wait for the sanctions to work, when they can develop nuclear weapons and force us to come to the table? They're not stupid.
Did Obama extract the best possible deal for the US? It's impossible to know without fully understanding the positions of the Europeans, Russians, and other players. If the US needs to maintain the sanctions at any cost, allies and frenemies alike will sniff it out and each extract their pound of flesh. They're not stupid, either.
We're talking about a weapon that the US first built in 1945, and a Princeton physics student nearly designed by himself in 1976. Today, the only real way to stop a proper country from obtaining one is to convince them that they're better off without it, but you're not going to convince somebody with little to lose. Obama's strategy to delay Iran's attempts and to eventually bring them back into the fold of world order seems plausible on its face.
Unless you want to go to war and occupy Iran indefinitely.